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MYOSIN & ASSOCIATED MUSCLE PROTEINS

M-line Muscle contraction   Skeletal   Smooth Myosin   Associated proteins   Disorders   Fiber types   Heavy chains   Light chains   Molecular regions   Other: Non-muscle   Structure Neuromuscular junction   Presynaptic   Postsynaptic Smooth muscle Thick filaments   Myosin   Non-myosin proteins Thin filaments   Actin   Regulatory proteins     Tropomyosin     Troponin   Structural components Z-line   Costameres Dystrophin & related proteins Muscle fiber structure & contraction

Skeletal muscle sarcomere Click on structure for more information

A-band: Entire length of thick filaments H-zone: Zone of thick filaments not associated with thin filaments I-band: Zone of thin filaments not associated with thick filaments M-line: Elements at center of thick filaments cross-linking them Z-Disc: Disc between I-bands

THICK FILAMENTS

• Myosins: General
  • Molecular motors
  • Interact with actin filaments: Utilize energy from ATP hydrolysis to generate mechanical force
  • Force generation: Associated with movement of myosin heads to tilt toward each other ³
  • Degradation
    • Mediated by ubiquitin-proteasome pathway (UPP)
    • Associated E3 ligases ubiquitylating MHC ¹¹
      • MuRF1: β/slow Cardiac; MHC-IIa Skeletal muscle; MBP C; Myosin light 1 & 2; α-actin
      • UBR2: MYH2B; MYH2X
• Myosin composition
  • Hexamer: Two trimers
    • Heavy chains: Homodimer
    • Each heavy chain binds 2 light chains
      • Essential light chain (ELC)
      • Regulatory light chain (RLC)
    • Molecular weight 520,000
  • Heavy chains; 2 types
    • Molecular weight: 220,000 each
    • C-terminal half: α helical
    • N-Terminal: Globular head
    • Skeletal muscle isozymes: Multiple types; Clustered on Chromosome 17p13.1
    • Cardiac isozymes (some also present in skeletal muscles): Chromosome 14
      
      

Fiber types ¹⁴: ATPase stain vs Myosin Heavy Chain (MYH) Types

Control Muscle MYH stains = Green; Collagen IV = Red

Myosin Heavy Chain Types: Human Muscle			Muscle Features: Other
Fiber   Type	Myosin: Heavy     Chain Types	Tissue	Disorders	Myonuclear Domain Size	Cytochrome Oxidase	NADH	PAS & Phosphorylase	Lipid	ATPase pH 4.3	ATPase pH 4.6	ATPase pH 9.4
I; Slow	I (MYH7)     (Cardiac β)	Skeletal muscle   MYH7: 6 wks to Adult Cardiac muscle   Fetal   Cardiomyopathy	MYH7   Myopathies	Moderate	High	High	Low	High	High	High	Moderate
IIa; Fast	IIA (MYH2)   ± I (MYH7)	Skeletal muscle   MYH2: 24 wks to Adult	MYH2   Myopathies	Larger	Moderate	Moderate	High	Moderate	Absent   or Low	Absent   or Low	High
IIb; Fast	IIA (MYH2)   + IIX (MYH1)   ± I (MYH7)	Skeletal muscle	MYH1   MYHM (Horse) ↓ #: ↑ Age; Female;   ↓ Handgrip; ↓ BMI * 18	Largest	Low	Low	Moderate	Low	Absent   or Low	Moderate	High
IIc	I (MYH7)         > IIA (MYH2)	Skeletal muscle   Immature fibers		Smaller	Low	High	Moderate	High	Moderate	Moderate	Moderate-   High
Fetal	IIB (MYH4)	Skeletal muscle   Regeneration, Early   Adult: Abnormal muscle   Other adult mammals:     2B fibers	Some LGMD	Small
Cardiac,   adult	Cardiac α     (MYH6)	Cardiac atria   Jaw muscle, Some	MYH6   Cardiomyopathies						Moderate	Moderate	Moderate
Congenital   Myopathy	I (MYH7)         + IIX (MYH1)	Skeletal muscle   Most fibers	Central core						Moderate	Moderate	Moderate
Other	Embryonic     (MYH3) 9	Skeletal muscle   Fetal, 6 to 24 wks   Adult: Regeneration     EOM; Spindle (Chain)	MYH3   Arthrogryposis
Other	Neonatal     (MYH8) 9	Skeletal muscle   Fetal, 7 wks to Birth   Adult: Regeneration     EOM; Spindle (Chain)	MYH8   Arthrogryposis
Super   fast	MYH13   (II eom)	Extraocular, Laryngeal Intrafusal
Slow twitch	MYH15	Extraocular
Slow tonic	MYH7B	Extraocular, Intrafusal   Tensor tympani
Smooth	Smooth muscle     (MYH11)	Aorta, Intestine	MYH11   Aneurysm (AAT4)   Visceral myopathy 2   MMIHS2
Other	MYH16	Jaw, Non-human
Other	Myosin 6   (MYO6)	Muscle Inner ear	Hearing loss
	Myosin 14	Ear Muscle spindles Extraocular muscle	Deafness PNMHH
	Myosin 15	Ear Muscle spindles Extraocular muscle	Deafness
	Myosin 18B   (MYO18B)	Muscle   Cardiac & Skeletal	Skeletal +   Myopathy
	Myosin 19   (MYO19)	Mitochondrial
Non-   muscle	Type A     (MYH9)	Fibroblast Endothelial cell Macrophage	MYH9   Hearing loss   Thrombocytopenia
	Type B     (MYH10)	Postsynaptic density Heart
	Myosin 7A	Ear	Hearing loss
	Myosin 5A	Skin Immune cells	Griscelli 1
	MYO9B	PNS & CNS	AR-CMT

• Myosin light chains: Types
  • Molecular weight: 20,000 each
  • Chemical classes: 2; Heads associated with 1 of each
    
    • DTNB light chain: Phosphorylatable; Regulatory
      • Heart (MYL2)
        • Diseases
      • CNS & Multisystem (MYL5)
      • MYL7
    • Alkalai light chain: Non-phosphorylatable
      
      • Skeletal muscle
        • Slow (MYL3)
          • Disease: CMH8
        • Fast (MYL1)
          • Disease: Congenital myopathy
        • Fetal (MYL4)
      • Smooth muscle
        • MYL6
  • Properties
    • Bind Ca⁺⁺: High affinity
    • Phosphorylated by myosin light chain kinase (MLCK)
      • Changes conformation of myosin heads
      • Increased population placed close to thin filaments
      • Potentiates actin-myosin interaction at low Ca⁺⁺ levels
      • Related enzymes:
        Myosin phosphatase target subunit 1 ;
        Guanosine triphosphatase Rho
    • Regulation of myosin's ATPase activity
    • Regulate myosin assembly into thick filaments
• Myosin molecular regions
  • Tail: α helical coiled-coil long thin rod
    • C-terminus half of heavy chain
    • Forms thick filament backbone
    • Self-association of distal 60% of rod (light meromyosin moiety)
    • Inter-myosin bonds are ionic
    • C-terminal residues: Non-helical; Affect myosin assembly into filament backbone
  • Head (S1 region): Globular
    • Molecular weight: 130,000
    • Composition
      • N-terminus of heavy chain
        
      • 2 Light chains: 1 of each class; Near junction with tail
    • Form cross-bridges
      • Arranged in helical pattern around thick filament
        
      • Bind actin when muscle contracts
    • Contains ATPase activity: 2 alkylated cysteine residues
    • Myosin in skeletal muscle has 2 heads; Other myosins may have only 1
    • Hinge region at tail-head junction
• Thick filament: Contains > 200 myosin molecules
• Myosin chaperone: unc-45B
• Myosin disorders
• Myosin: Other
  • Also see: Unconventional myosins
  • External link

Non-myosin components in thick filament C-proteins (MYBPC) 12 Structure: Single polypeptide chain; Molecular weight 140,000 Location Middle 1/3 of each half of A-band C region: Cross-bridge-bearing zone Binds to: Myosin tail region Maintains thick filaments in bundles of 200 to 400 molecules Types Slow (MYBPC1) Fast (MYBPC2) Cardiac (MYBPC3) Diseases/Disorders MYBPC1 MYBPC2 Distal arthrogryposis 17 Immunization 1 Peptide: Amino acids 284–580; 965–991 Potent induction of experimental immune myositis (EAM) Knock-out mouse: Altered 16 Length-dependent activation Myofilament structure MYBPC3 Hypertrophic cardiomyopathy (CMH4) + Myopathy Dilated cardiomyopathy 1MM (CMD1MM) Left ventricular noncompaction 10 (LVNC10) H protein (MYBPH) Increased in muscle in ALS Titin Largest protein known: Molecular weight 3,000,000 Major domains Immunoglobulin ~300 copies Located in both A band (160 copies) & I band Fibronectin ~300 copies Not prominent in A band Phosphorylation sites Near N & C-terminus Associated with regulation of myogenesis Kinase domain Located at edge of M-line Phosphorylates telethonin Elastic region of titin: Links thick filament to Z line Early phase of lengthening involves titin straightening Further lengthening involves unfolding of peptide in I band regions Gives rise to extensible properties of muscle Z-disc: Z repeat; Associated with telethonin Location in muscle: Spans sarcomere from M line to Z line Isoforms Several: Change during development Most very large size Largest isoform: Soleus Smaller isoform: Psoas 1 small isoform: Novex-3 (Xtn3) Developmentally expressed in heart & skeletal muscle Titin I-band region Binding Z-line: Telethonin (TCAP); α-actinin I-band: Calpain-3 (CAPN3); Obscurin (OBSCN) M-line: Myosin-binding protein C (MYBPC3); Calmodulin (CALM1); CAPN3; MURF1 Functions Directs assembly of contractile elements Provides sarcomere elasticity via Serial spring elements Prevents over-stretch of sarcomere ? Myofibrillar cell signalling Kinase domain: Carboxy-terminal Phosphorylation sites: Associated with regulation of myogenesis Calpain 3 protease-binding motif Disease associations Hereditary Immune disorders Dermatomyositis: Titin reduced in perifascicular, atrophic muscle fibers Myasthenia gravis: MGT-30 antibodies Sporadic ALS: Low levels associated with rapid disease progression Mouse: Muscular Dystrophy with Myositis (mdm) Other subclass of titin-like molecules Present in smooth muscle Smaller: 800 kD Twitchins, Projectins, Mini-titins X protein

MyBPC1 labeled with GFP: Localizes to C zones in A-band From: C Weihl MD

• Function: Stabilizes thick-filament lattice
• Components: Contains 3 proteins
  • Creatine kinase
  • M protein (165 kD)
  • Myomesin (190 kD) : Binds to myosin, titin & light meromyosin

• Myopathy: Acquired
  • Myosin-loss (Acute quadriplegic; Critical illness) myopathies: Myosin loss from muscle fibers
• Myopathy: Hereditary
  • MYH2 (MYHC IIa): IBM3
  • MYH7 (MyHC I): Myopathies & Cardiomyopathies
  • MYL2: Dilated cardiomyopathy with type 1 muscle fiber hypotrophy, Recessive
• Motor neuropathy, Distal (dHMN)
  • MYH14: Distal Weakness, Hoarseness, Hearing loss (PNMHH)
• Cardiomyopathy
  • Hypertrophic cardiomyopathies
    • MYH7
      • CMH 1
      • Hypertrophic with Cores
    • MYH6: CMH 14
    • MYL2: CMH 10
    • MYL3: CMH 8
    • Myosin light chain kinase 2 (MYLK2): Cardiomyopathy
    • Cardiac myosin binding protein-C (MYBPC3): CMH 4
  • Dilated cardiomyopathies
    • MYH7: CMD 1S
    • MYH6: CMD 1EE
  • Left ventricular non compaction: MYH7
  • Atrial fibrillation 18, Early onset, Dominant : MYL4 ⁷
• Distal arthrogryposis
  • MYH8
    • Distal arthrogryposis with cardiac myxomas
    • Distal arthrogryposis with trismus pseudo-camptodactyly (DA7)
  • MYH3: Distal arthrogryposis
    • Freeman-Sheldon (DA2A)
    • Sheldon-Hall (DA2B)
    • DA8
• Hearing loss
• Other myosin syndromes
• Also see: Myosin & Fiber types

THIN FILAMENTS

Actin Regulatory proteins   Tropomyosin   Troponin   Profilins   Other Structural components Z-line Costameres

• Most abundant protein in cytoplasm of mammalian cells
• G-actin (Globular)
  • Spherical monomer
  • Present at low ionic strength
  • 2 domains: Large & small
  • Nucleotide (ATP or ADP) binding in cleft between domains
  • Each actin can bind to 4 other actins
  • G-actin molecule contains a high-affinity myosin head binding site
  • 2 α types expressed in muscle: Skeletal (ACTA1) ; Cardiac (ACTC1)
• F-actin
  • Helical polymer
  • Self associates
    • Head to tail polymerization of asymmetric monomers
    • At physiologic ionic strength
    • Hydrolysis of ATP to ADP speeds polymerization & imposes polarity
    • Steady-state flux of subunits through F-actin
      • Net assembly at plus ends: Nearest the peripheral membrane; ATP-actin monomers added
      • Net disassembly at minus ends: Cytoplasmic end; ADP-actin monomers released
  • Some actin-binding proteins inhibit polymerization
    • Cytosolic
      • Profilin 1 , and 2 : Concentrated at regions of new actin filament assembly
      • Thymosin β-4
      • Gelsolin : Severs actin filaments in presence of submicromolar calcium; Solates cytoplasmic actin gels
    • Z-line: Capping proteins inhibit polymerization at the most active, barbed (plus) end
  • Other actin filament associated proteins
    • Actin bundling proteins: Plastins
    • α-actinin
    • CAP2
    • Nexilin
    • Nebulin
    • Tropomyosin
    • Cofilin-2
    • LMOD3
    • XIRP2 (CMYA3)
• Actin filaments
  • Contain ~360 actin molecules
  • 2 strands of F-actin twisted to form a double helix
  • Flexible along long axis
  • G-actin binds myosin heads
  • Actin capped & thin filament stabilized by β-actinin
• Actin Disorders ⁶
  • ACTA1 : Skeletal muscle actin
    • Genetics
      • ACTA1 mutations: ~200 described; High new mutation rate
      • Inheritance: 90% Dominant; 10% Recessive
      • De novo mutations
        • Often dominant
        • Associated with somatic or gonadal mosaicism: Some patients
    • Clinical: ACTA1 disorders
      • Extraocular muscles: Always spared
      • Heart: Usually spared
      • Amount of mutant ACTA1 in skeletal muscle: Influences dominant ACTA1 disease severity
    • Skeletal muscle actin (ACTA1): Clinical syndromes
      • Dominant
        • Congenital myopathy with excess of thin filaments
        • Nemaline rod myopathy (NEM3)
        • Intranuclear rod myopathy
        • Congenital fiber type dysproportion (CFTD)
        • Cap myopathy
        • Inclusion body, vacuolar myopathy with CFTD
        • Cytoplasmic body myopathy
        • Distal myopathy
        • Rod myopathy with cores
        • Myofibrillar myopathy
          • Congenital
          • Child
        • Scapuloperoneal myopathy
        • Zebra body myopathy
      • Recessive
        • Nemaline rod myopathy
        • Congenital myopathy with Rigid spine
  • ACTC1 : Cardiac & Skeletal muscle actin ¹³
    • Dilated cardiomyopathy 1R
    • Cardiomyopathy, familial hypertrophic 11 (CMH 11)
    • Atrial septal defect 5
    • Left ventricular noncompaction 4
    • Distal arthrogryposis + Congenital heart defects
  • Actin-α2, Smooth muscle, aorta (ACTA2)
    • Aortic aneurysm, familial thoracic 6
    • Moyamoya disease 5
    • Multisystemic smooth muscle dysfunction syndrome
  • Actin-β (ACTB)
    • Baraitser-Winter syndrome 1
    • Dystonia, juvenile-onset
  • Actin-γ1 (ACTG1)
    • Baraitser-Winter syndrome 2
    • Deafness, autosomal dominant 20/26
  • Actin-γ2 (ACTG2)
    • Familial visceral myopathy
  • Actin-related protein: Adducin 3
    • Cerebral palsy, Spastic Quadriplegic (CPSQ)

• Tropomyosin
  • General function: Regulates, with troponin, Ca⁺⁺ dependent muscle contraction
  • Structure
    • 2 stranded α-helical coiled-coil protein
    • α and β subunits
  • Distribution
    • Slow twitch muscle fibers: β-tropomyosin (TPM2) predominant
    • Fast twitch muscle fibers: α-tropomyosin (TPM1) predominant
  • Interactions
    • Forms dimer
      • Most common: αβ
      • Other: α2; β2
    • Each tropomyosin dimer interacts with 7 actin molecules
    • β-Tropomyosin: Coiled-coil protein; Binds head to tail along length of actin filament
    • Controls muscle contraction
      • Inhibits actin-myosin interaction in Ca⁺⁺ senstive manner
      • Relaxed state: Probably interferes with actin-myosin interaction
  • Disease mutations
    • α1 subunit (TPM1): Cardiomyopathy
      • Hypertrophic (CMH3)
      • Dilated (CMD 1Y)
    • α-tropomyosin-3 (TPM3) : Slow skeletal
      • Disorders
    • Tropomyosin 2 (β-Tropomyosin; TPM2) : Skeletal muscle beta
      • Allelic disorders
      • Lower levels in atherosclerosis models
  
  
• Troponin: Complex of 3 subunits
  • General function: Prevents actin-myosin interaction in resting muscle
  • TN-I ⁸
    • Function
      • Inhibitory subunit of troponin complex in sarcomeric thin filament
      • Inhibits actin-myosin interaction
      • Role in calcium regulation of contraction & relaxation
    • Types
      • Skeletal muscle
        • Slow (TNNI1)
        • Fast (TNNI2) : Distal arthrogryposis, Type 2B
      • Cardiac (TNNI3)
        • Disorders: Hypertrophic (CMH7) & Restrictive cardiomyopathies
    • Related molecule: TNNI3K
      • MAPKKK family of protein kinases
      • Disorder: Cardiac conduction disease with or without dilated cardiomyopathy (CCDD)
  • TN-C
    • Skeletal muscle types
      • Slow (TNNC1)
        • Disorders
          • Dilated Cardiomyopathy 1Z ;
          • Hypertrophic cardiomyopathy 13
      • Fast (TNNC2)
        • Disorders
          • DM-VP: TNNC2 Reduced in perifascicular atrophic muscle fibers
          • MYONRI
    • Function: Binds Ca⁺⁺ at Low & high affinity sites
    • One of most highly conserved proteins known
  • TN-T
    • Types: 3 genes expressed in tissue-specific manner
      • Skeletal muscle
        • Slow (TNNT1)
        • Fast (TNNT3)
      • Cardiac troponin T2 (TNNT2)
    • Function: Tropomyosin binding subunit
    • Disorders
      • TNNT1: Nemaline rod myopathy 5 (NEM5; Amish type)
      • TNNT2: Hypertrophic cardiomyopathy 2; Dilated cardiomyopathy 1D
        • Mechanisms of action: May vary with specific gene defect
          • Reduced Calcium sensitivity of force production: Increased Energetic load on the heart
          • Increased Unloaded shortening velocity: Increased Energetic load on the heart
          • Dominant-negative effect: Blocks Ca⁺⁺ activation of thin filament
      • TNNT3: Distal arthrogryposis 2B (DA2B)
  • Other disease relations
    • TN-T & TN-I levels increased in blood after myocardial infarction
• Profilins
  • Distribution: Present in 1:1 complex with monomeric actin
  • Actions
    • Bind to PIP2: Inhibits formation of IP3 & DG
    • Low concentration: Enhance actin polymerization
    • High concentration: Inhibit actin polymerization
  • Profilin 1
    • Present in many cells
    • Cell locations
      • Cytoplasm: Present in 1:1 complex with monomeric actin
      • Mitochondria
    • Embryogenesis: Effects on cell division & survival
    • Disorder: Familial ALS
  • Profilin 2
    • Distribution: Highest levels in brain, skeletal muscle & kidney
  • Profilin 3
  • Profilin 4
• Cofilin-2
  • Skeletal muscle specific isoform
  • Localized to thin filaments
  • Exerts effect on actin, in part, through interactions with tropomyosins
  • Disorder: Nemaline rod myopathy 7
• Smooth & non-muscle cells also have
  • Caldesmon
    • Lies in long pitch grooves of actin helix
    • Parallel and associated with tropomyosin
  • Calponin

• Nebulin
  • Properties
    • Large protein: 600,000 to 900,000 kD isoforms
    • Abundant: 3% to 4% of myofibrillar protein
    • Contains 179 to 239 copies of a 35- to 40-amino acid module (Simple repeats)
    • Actin binding: May bind > 200 actin monomers
    • Many isoforms
  • Cellular location: Thin filament
    • Z disk & A bands
    • Extends whole length of thin filament
    • C-terminal: Z-disk anchor
    • 2 nebulin molecules per thin filament
  • Tissue
    • Skeletal muscle
    • Cardiac: Especially atrial cardiomyocytes
  • Function
    • Probably acts as inextensible template for thin filament assembly
    • Binds & Stabilizes actin filaments in sarcomeres
    • Restricts thin filament length
    • Stiffens thin filament
    • Generates physiological force levels
  • Interacting proteins
    • Actin
    • Tropomyosin
    • CapZ
    • Tropomodulin (TMOD)
    • Desmin
    • Titin
    • α-Actinin
    • KLHL40
    • KLHL41
    • WASL (N-WASP)
    • Xin (XIRP1; CMYA1)
  • Diseases
• Other
  • Nebulin-related anchoring protein
    • Location
      • Skeletal muscle: Myotendinous junction
      • Cardiac muscle: Intercalated disc
    • Function: ? Anchors terminal actin filaments to subsarcolemmal protein complexes
  • Nuclear Factor of Activated T Cells (NFATc): Translocates to nucleus with electrical stimulation
• Z line components & structure ¹⁰
  • Anatomy: Disc between I-bands
  • Size
    • Thin in fast muscle
    • Thick in slow muscle
  • Disorders
  • α-Actinin
    • Structure
      • Homodimer: 97 kD subunits; Arranged in anti-parallel direction
      • Similar to spectrin
    • Muscle specific: Types 2 & 3
      • Type 2 (ACTN2)
        • Expression
          • High in Cardiac & Skeletal muscle
          • Present in all muscle fibers
        • Active as dimer
        • Allosteric regulation: By phosphoinositides (Phosphatidylinositol bisphosphate)
        • Binds & Crosslinks: Anti-parallel actin filaments
        • Anchors cytoskeletal & sarcomeric proteins
          • Myopalladin
          • Myotilin
          • Muscle LIM protein
          • Titin: Anchors titin within Z-disk
          • Nebulin
        • Link between sarcomere & membrane: Interactions with
          • Vinculin
          • Integrins
          • Dystrophin complex
          • GNE
          • ZASP
        • Other functions
          • Ion channel regulation
          • Gene expression
          • Signalling cascades
          • Mechanosensor
      • Type 3 (ACTN3)
        • Expression
          • Skeletal muscle
          • Present in subset of type II fibers
        • Interacts with: Calsarcin; Calcineurin
      • Located only in: Z line; Costameres
      • Ca⁺⁺ insensitve
      • Form strong complex with F-actin: Anchor actin filaments
      • Also present in
        • Smooth muscle: Component of dense bodies & Plaques
        • Cardiac muscle
    • α-actinin disorders
      • α-actinin is: Major component in rods
      • α-actinin-1 (ACTN1) : Macrothrombocytopenia
      • α-actinin-2 (ACTN2)
        • Dilated cardiomyopathy 1AA (CMD 1AA)
        • Hypertrophic cardiomyopathy 23 (CMH23)
        • Multiple structured Core Disease
        • Distal myopathy
      • α-actinin-3 (ACTN3) Nonsense mutations
        • Heterozygotes: Very common in normal populations
        • Homozygotes: Arg577Ter (XX) mutation
          • Variable frequency in different populations
            • Europeans: 18%
            • Asians: 25%
            • African Bantu: < 1%
            • Athletes
              • Lower frequency of Ter (XX) mutation than controls
                • Elite sprint: Absent in Females
                • Elite strength athletes: Bodybuilders & Powerlifters
              • Higher frequency of Ter (XX) mutation
                • Endurance athletes
          • No clinical disease phenotype
          • Normals: ? Reduced power generation
          • Age with XX
            • Gait slowing: More
            • Sarcopenia worse
          • Muscular dystrophy: DMD patients less function
          • Serum CK: Lower
        • Higher frequency of 577R allele than controls
          • Elite sprint athletes: Male & Female
          • Power athletes: Male
        • R577X: More severe phenotype
          • Acid maltase deficiency
          • Duchenne dystrophy
        • Knockout mouse
          • Glycogen phosphorylase (GPh; PYGM) : Reduced activity
          • Increased glycogen in muscle
          • Force generation: Reduced
          • Fast fiber diameter: Reduced
          • Aerobic metabolic pathway: Increased activity
          • Enhanced recovery from fatigue
      • α-actinin-4 (ACTN4) : Focal segmental glomerulosclerosis 1
    • α-actinin in non-muscle cells
      • Epecially actin 1 & 4
      • Cytoskeletal isoform
      • Ca⁺⁺ sensitve
      • Location: Along microfilament bundles & adherens-type junctions
      • Function: Binds actin to the membrane
  • Desmin
    • Circles periphery of Z line
    • Functions
      • May link Z lines of adjacent myofibrils
      • Integrates the contractile apparatus with sarcolemma & nucleus
    • Muscle-specific member of the intermediate filament family
    • Early myogenic marker in skeletal muscle & heart
      • Precedes MyoD family of myogenic helix-loop-helix regulators except Myf5
    • Disorders
      • Myofibrillar myopathies: Normal desmin in aggregates in muscle fibers
      • Desmin mutations: Myofibrillar myopathies
  • Desmuslin
    • Type VI intermediate filament protein
    • Colocalized with desmin
    • Interacts with α-dystrobrevin
    • Linkage between extracellular matrix & Z-discs: Structural support for muscle cell
    • Expressed in heart & Skeletal muscle
  • Capping proteins: α1 ; α2 ; β subunits
    • Caps the barbed end of actin filaments
    • Regulates the growth of the actin filament at the barbed end
  • Calpain 3: LGMD 2A
  • Calcineurin: Binds to calsarcins
  • Calsarcin 1 (Myozenin 2) : Adult cardiac & slow-twitch skeletal muscle
  • Calsarcin 2 (Myozenin 1)
    • Fast skeletal muscle
    • Binds to filamin C
  • CapZ : β1 isoform
  • Myopalladin (MYPN)
    • Tethers nebulin in skeletal muscle to α-actinin at Z lines
    • Also present in I-band of cardiac muscle & nucleus
    • Diseases: CMD 1KK; CMH22; RCM4
  • Myosin 18B (MYO18B)
  • Myotilin (TTID) : Interacts with α-actinin
  • Nebulin (NEB) : C-terminal region
  • Supervilin : Actin binding protein; ? Link with membrane
  • Telethonin : LGMD 2G
  • Nexilin (NEXN) : Dilated cardiomyopathy
• Costameres ⁴
  • Location
    • Subsarcolemmal
    • Circumferentially align in register with the Z disk
  • Functions
    • Transduce forces generated by myofibrils
      • Across sarcolemma
      • To extracellular matrix & other muscle cells
    • Coordinate organized folding of the sarcolemma
      • Minimizes stress on sarcolemmal during muscle contraction or stretch
    • Homology: Focal adhesions expressed by non-muscle cells
  • Components
    • Aciculin
    • γ-Actin
    • α-Actinin
    • Ankyrins
    • CapZ : β2 isoform
    • Collagens: Types IV & XIII
    • Chisel (Csl/Smpx) : Upregulated in response to passive stretch
    • Desmin: Physical link between Z-disk & sarcolemma
    • Dystroglycan
    • Dystrophin
      • Coupling between sarcolemma and γ-actin filaments of costameres
      • Loss of dystrophin leads to disorganized costameric lattice
    • Integrins: α7 & β1
    • Laminin-2
    • Melusin : Prevents cardiac dilation during chronic pressure overload by sensing mechanical stress
    • Muscle-specific LIM Protein (MLP): Dilated cardiomyopathy 1M
    • Neuronal Nitric Oxide Synthase (nNOS)
    • Raver1
      • Localized to costamere in adult skeletal muscle
      • Redistributes from nucleus to cytosol during myotube formation/differentiation
    • Spectrin
    • Syncoilin
    • Synemin
    • Syntrophin
    • Talins : Links vinculin to integrins
    • E-Tropomodulin (E-Tmod)
    • Type-II SH2-domain containing inositol 5-phosphatase (SHIP2)
    • Vimentin
    • Vinculin
      • Focal adhesion protein
      • Vinculin splice variant (metavinculin) mutations cause dilated cardiomyopathy

SKELETAL MUSCLE CONTRACTION & RELAXATION

• Myosin binding sites on actin are obscured
  • 1 Tropomyosin covers the myosin binding sites of 7 G-actin residues
  • Tropomyosin resides in grooves of F-actin helix
• Myosin in high-energy conformational state
• High energy state produced by ATP

1. Sarcolemma depolarization: Spreads to internal T tubule system
2. Cytosolic Ca⁺⁺ Increased
   • Voltage-gated Ca⁺⁺ channels in SR membranes opened
   • Movement of cisternal Ca⁺⁺ into the sarcoplasm
   • Contraction when Ca⁺⁺ concentration > 1 to 5 μM
3. Myosin binding sites on actin become available
   • Troponin subunit C binds Ca⁺⁺
   • Troponin undergoes conformational change
   • Attached tropomyosin moved
     • More deeply into helix groove of F-actin
     • Away from myosin binding sites on actin
     • Uncovers myosin binding sites on G-actin subunits
     • G-actin subunits available to interact with myosin headpieces
4. Actin-myosin complex formed
5. Binding influenced by phosphorylation of myosin light chain
6. Dissociation of Pi and ADP
7. Conversion of myosin to low-energy conformational state
8. Rapid myosin conformational changes about its hinge points
9. Translocation of attached thin filament toward the M line of sarcomere

1. Sarcoplasmic membrane & T tubule return to resting potential
   • ~ 60 mV more positive outside
2. Sarcoplasmic Ca⁺⁺ reduced
   • Pumped into the SR cisternae by ATP-driven Ca⁺⁺ pump
   • For each ATP hydrolyzed, 2 Ca⁺⁺ ions moved out of the sarcoplasm
   • Calsequestrin , on cisternal surface of the SR membrane, binds Ca⁺⁺
   • Binding decreases Ca⁺⁺ concentration in cisternae: Favors more Ca⁺⁺ accumulation
   • Sarcoplasmic Ca⁺⁺ reduced to < 0.1 μM
     • 50-fold < KD for calcium binding to troponin-C
3. Sarcoplasmic Ca⁺⁺ also sequestered in mitochondrial matrix
   • Via mitochondrial Ca⁺⁺ pump
   • Driven by electron transport-generated chemiosmotic potential
   • Aerobic conditions:
     • Pump uses electron transport energy to sequester calcium
     • Occurs in preference to ATP synthesis
4. Original conformational states of troponin and tropomyosin recur
5. Interaction between actin & myosin prevented

	Type 1	Type 2
Color	Red	White
Twitch speed	Slow	Fast
Action	Tonic	Phasic
Oxidative enzymes	High Aerobic	Low Anaerobic
Mitochondria	Many	Few
Lipid content	Moderate	Low
Glycogen content	Low	Moderate
ATPase staining   pH 9.4	Light	Dark
ATPase staining   pH 4.3	Dark	Light

SMOOTH MUSCLE

• No organized sarcomere structure
• Thin & thick filaments present in random, disorganized fashion
• Sarcoplasmic reticulum: Poorly developed and sparse
• Cytoplasmic Ca⁺⁺: Source is extracellular fluid
• Caldesmon substitutes for troponin subunits
  • Binds to tropomyosin & actin
  • Inhibits myosin heads from moving
  • Inhibition ↓ by:↓
    • Protein kinase C (PK-C): Phosphorylates caldesmon, or
    • Ca⁺⁺-Calmodulin complex: Binds to caldesmon & releases it from actin
  • ↓ inhibition produces contraction
    • Tropomyosin changes location in helical grooves of F-actin
    • Actomyosin ATPase activity is stimulated.
  • Calponin: Also inhibits myosin movement
    • Binds to actin
    • Calponin phosphorylation reduces actin binding
      • By Calmodulin-dependent kinase II or PK-C: Activated by Increased Ca⁺⁺
      • Calponin dephosphorylation: Rebinding to actin; Reduced myosin movement
• Myosin regulation
  • Smooth muscle myosin contains a regulatory light chain
  • Myosin light chain kinase (MLCK)
    • Contains large & small (calmodulin) subunit
    • Activated by association of 4 Ca⁺⁺ ions with calmodulin subunit
    • Activated MLCK
      • Phosphorylates myosin regulatory chain
      • Stimulates ATPase activity
    • MLCK downregulated by
      • Phosphorylation of large subunit
      • Rise in cAMP & activation of a cAMP dependent protein kinase
    • Exaggerated CK post-exercise (C49T & C37885A)
  • Hormonal activation of PK-C
    • Phosphorylates myosin light chains: At different site than MLCK
    • Inhibits ATPase activity
  • Regulation of myosin activity in smooth muscle is slow

OTHER (UNCONVENTIONAL) MYOSINS

• Actin-based motor molecules with ATPase activity
• Axonal transport: In areas near synapses

• Myosin 3A
  • DFNB30
• Myosin 6 (MYO6)
  • Locations
    • Inner ear hair cells
    • Muscle
      • Normal: Sarcoplasmic reticulum; Nuclei; Neuromuscular junction
      • Denervation & Atrophy: Increased levels
  • Functions
    • Axon transport
    • Minus-end-directed trafficking for endocytic trafficking
    • Internalization of TrkB & AMPAR
  • Disorders
    • DFNB37
    • DFNA22 ± Hypertrophic cardiomyopathy
    • Mouse model: Snell's walzer
• Myosin 7A (MYO7A)
  • Function: Microtubule organization for morphogenesis of inner ear sensory cell stereocilia
  • Deafness, non-syndromic (DFNB2)
  • Usher syndrome 1B : Congenital deafness; Vestibular areflexia; Progressive retinitis pigmentosa
  • shaker-1 mouse
• Myosin 9A (MYO9A)
  • Disorder: Congenital myasthenia gravis
• Myosin heavy chain A (MYH9)
  • Fechtner syndrome
  • Hematologic: Thrombocytopenia; Large platelets; Leukocyte inclusions ('Döhle-like' bodies)
  • Sensorineural deafness: Alport-like
  • DFNA 17
• Myosin 14
  • Deafness, Autosomal dominant (DFNA4)
  • PNMHH
• Myosin 15 (MYO15A)
  • Function: Targeting of whirlin to tips of stereocilia
  • Deafness with vestibular defects: Congenital Recessive (DFNB3)
  • Mouse model: Shaker-2

• Myosin 1A
  • Functions
    • Cell migration
    • Neurite outgrowth
    • Vesicular transport
    • Molecular force sensor
  • Disease: Deafness, dominant 48 (DFNA48)
• Myosin 1B : Movement of organelles along actin filaments
• Myosin 1C : Stereocilia adaptation; GLUT4 traffic
• Myosin 1D : Drives gliding of actin filaments in circular, counterclockwise paths in vitro
• Myosin 1E
  • Podocyte membrane in renal glomerulus
  • Disease: Childhood Familial Focal Segmental Glomerulosclerosis
• Myosin 1F : Immune system
• Myosin 1H: CCHS2
• Myosin 5A (MYO5A; MYH12)
  • Protein
    • Unconventional myosin
    • Functions
      • Short distance transport of vesicles
      • Intracellular, actin-based transport & secretion
      • Interacts with: ANKFY1
  • Griscelli syndrome 1 (GS1) (Elejalde disease )
    
    • Genetics: Recessive
    • Clinical
      • "Neuroectodermal melanolysosomal disease"
      • Silvery hair
      • Pigment abnormalities (Hypomelanosis)
      • CNS: Seizures; Severe hypotonia; Mental retardation
      • No immune or hemophagocytic syndrome
    • Mouse model: 'Dilute lethal'
    • Also see: Griscelli syndrome 2
  • Mouse: Ataxia ¹⁵
    • Genetics
      • Spontaneous
      • Mutation: c.1752G>A)
      • Recessive
    • Clinical
      • Ataxia
      • Failure to wean
    • Pathology
      • Neurons: Mitochondria Elongated
      • PAX6-cells: External cerebellar granule layer
• Myosin 5B
  • Actin filament-based motor protein
  • Binds some GTPase RAB proteins, including RAB8
  • Functions: Transport
    • Recycling endosome motility
    • AMPAR traffic
  • Disease: Microvillus inclusion disease + Diarrhea
• Myosin 7B
• Myosin heavy chain A (MYH9)
  • May-Hegglin anomaly : Macrothrombocytopenia
• Myosin 9B (MYO9B)
  • Functions
    • Minus-end-directed motility
    • Actin-based processes in myeloid cells
  • Location: Leukocytes; PNS; CNS
  • Diseases
    • AR-CMT2
    • Susceptibility to: Celiac disease
• Myosin 10 (MYO10)
  • Associates with regions of dynamic actin
  • Mouse knockout: Fewer and larger myocytes
  • Functions: Transport
    • Growth cone turning & protruding motility of spines
    • Cytokinesis
• Myosin, heavy chain 11, Smooth muscle (MYH11)
  • Visceral Myopathy 2 (VSCM2) Chronic intestinal pseudoobstruction (CIPO)
    ● Myosin, heavy chain 11, smooth muscle (MYH11) ; Chromosome 16p13.11; Dominant
    • Epidemiology: 3 families
    • Genetics
      • Inheritance: Dominant
      • Mutations: Stop; Deletion
      • Allelic disorders
        • Megacystis-microcolon-intestinal hypoperistalsis syndrome 2 (MMIHS2)
        • Aortic aneurysm, familial thoracic 4
        • Acute myeloid leukemia of M4EO subtype: Chromosomal aberration involving MYH11
    • MYH11 protein
      • Myosin, heavy chain
      • Smooth muscle
    • Clinical
      • Onset age: Child or Adult
      • Chronic intestinal pseudoobstruction
      • Smooth muscle dysmotility: Esophageal, Gastric, Intestinal, Anal
      • Dysphagia
      • GERD
• Myosin 16
  • Neuronal tyrosine-phosphorylated phosphoinositide 3-kinase adaptor 3 (NYAP3)
  • Location: Neocortex & Olfactory bulb
  • Phosphorylated by Fyn after stimulation with Contactin-5 (CNTN5)
  • Actin remodeling
• Myosin 18B

• RAB27A protein
  • Effector of cytotoxic granule exocytosis
  • Interacts with MYO 5A
• Clinical
  • Pigmentary dilution
    • Skin: Diffuse pigmentation
    • Hair: Silvery; Clumps of pigment in shafts
    • Melanocytes: Accumulation of melanosomes
  • Immunologic
    • Absent delayed-type cutaneous hypersensitivity
    • Acute phases of uncontrolled lymphocyte and macrophage activation
    • Impaired natural-killer cell function
    • Hemophagocytic syndrome
  • Neurological
    • Onset after immune disorders
    • Due to lymphocytic and histiocytic infiltration of the brain
  • Prognosis: Death unless bone marrow transplantation
• Mouse model: ashen
• Also see: Griscelli syndrome 1 (Elejalde syndrome)