Myosin

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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

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

Myosin

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

Images
Myosin HC 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)	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
	MYO9B	PNS & CNS	AR-CMT
Also see Fiber type disorders Myosin disorders

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:
  - 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
- Located in middle 1/3 of each half of A-band
- Binds to myosin tail region
- Maintains thick filaments in bundles of 200 to 400 molecules
- Types
  - Slow (MYBPC1)
  - Fast (MYBPC2)
  - Cardiac (MYBPC3)
- Diseases
  - MYBPC1
  - MYBPC3
    - Hypertrophic cardiomyopathy (CMH4) + Myopathy
    - Dilated cardiomyopathy 1MM (CMD1MM)
    - Left ventricular noncompaction 10 (LVNC10)
  - Immunization with C-protein: Induces experimental immune myositis ¹
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

M line

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

Myosin: Disorders ⁵

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

I-band: Zone of thin filaments not associated with thick filaments

Actin

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)

Regulatory proteins of thin filament

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

Thin filaments: Structural components & Associated structures

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)
- Cellular location: Thin filament
  - Z disk & A bands
  - Extends whole length of thin filament
  - 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
- 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

Resting muscle

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

Muscle contraction

Sarcolemma depolarization: Spreads to internal T tubule system
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
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
Actin-myosin complex formed
Binding influenced by phosphorylation of myosin light chain
Dissociation of Pi and ADP
Conversion of myosin to low-energy conformational state
Rapid myosin conformational changes about its hinge points
Translocation of attached thin filament toward the M line of sarcomere

Muscle relaxation

Sarcoplasmic membrane & T tubule return to resting potential
- ~ 60 mV more positive outside
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
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
Original conformational states of troponin and tropomyosin recur
Interaction between actin & myosin prevented

Muscle Fiber Contractile Types

	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

Structure & Function

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

Myosins: General properties

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

Hearing loss with abnormal unconventional myosin heavy chains

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 ('D�hle-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

Other myosins & syndromes

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)
- Functions: Short distance transport of vesicles
- Griscelli syndrome 1 (GS1) (Elejalde disease )
  - "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
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

Griscelli syndrome 2 (GS2)

²
● RAS-associated protein RAB27A (RAB27A)

; Chromosome 15q21.3

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)

Return to Myopathies
Return to Neuromuscular home page

References
1. J Neuroimmunol 1999;98:130-135
2. Am J Hum Genet 2002;August
3. PNAS 2002;99:14801�14806
4. J Biol Chem 2003;Online Jan 29
5. Neuromuscul Disord 2007; Apr 12
6. Acta Neuropathol 2012; Online July
7. Nat Commun 2016;7:11303
8. Gene 2016;576:385-394
9. Skelet Muscle 2015;5:22
10. Int J Mol Sci 2021;22:3058
11. Proc Natl Acad Sci U S A 2022;119:e2200215119
12. Front Genet 2022;13:896117
13. HGG Adv 2023;4:100213
14. Cold Spring Harb Perspect Biol 2024:a041477

8/14/2024