L6.3 Key functional protein in skeletal muscles

Question 1

What is sarcomere?

Answer 1

Basic functional unit of a striated muscle

Question 2

What is a costamere?

Answer 2

specialised sites of transmembrane complexes → transmission of force concentrated

Question 3

What are the 2 protein complexes for transmitting force?

Answer 3

• Dystrophin-glycoprotein complex (DGC)
• Integrin-talin-vinculin complex

Question 4

What is the DGC complex composed of?

Answer 4

• Dystrophin
• Dystroglycan complex
• Sacroglycan complex

Question 5

Characteristics of dystrophin

Answer 5

• Rod like protein (rigid, but flexible at 4 hinge points)
• Stabilises DGC
• 2 actin binding domains:
  
  • N-terminal (calponin homology domains)
  • Centrally located spectrin like repeats (24 repeats → creates rod like structure)
• Cysteine rich domain (Sites of protein-protein interaction)
• C-terminal domain (interacts with syntrophin & dystrobrevin)

Question 6

Function of dystrophin

Answer 6

• Links sacrolamma to actin cytoskeleton
• Signalling
  
  • Syntrophin & dystrobrevin
  • nNOS → produce NO for vasodilation

Question 7

Absence of dystrophin

Answer 7

• Duchenne’s/Becker’s muscular dystrophy
  
  • No dissipation of force → creates small tears in membrane → permeable → ↑muscle fibbre breakdown & regen/muscle death

Question 8

What is the dystroglycan complex?

Answer 8

• composed of α & β dystroglycan & laminin
• Links extracellular matrix to plasma membrane
• Links actin cytoskeleton of muscle fibres (by interacting with dystrophin)

Question 9

Characteristics and function of Laminin

Answer 9

• Large, EC, trimeric protein
• α (only one needed for interaction), β & γ chains
• Function:
  
  • Links EC matrix & dystroglycan (maintains structure)

Question 10

Mutation of Laminin

Answer 10

• Congenital muscular dystrophy MCD1A
  
  • No α chain (merosin)
  • Presence of immature fibres (↑ connective tissues - fibrosis)

Question 11

Function of dystroglycan

Answer 11

• Transmembrane linker
• NEEDS post-translational modification for proper function
• Function:
  
  • α: has multiple sites for glycosylation (receptor for laminin)
  • β: transmembrane → binds to α-dystroglycan & dystrophin
  • Provides link between subsarcolemmal cytoskeleton & EC matrix

Question 12

Mutation of dystroglycan

Answer 12

• No mutation in dystroglycan itself
• But have mutations in glycosyltransferases → affects post-translational function

Question 13

Characteristic and function of Sarcoglycan

Answer 13

• α, β, γ, δ, sarcospan subunits
• Function
  
  • Stabilises DGC

Question 14

Absence and mutation of sarcoglycan

Answer 14

• Absence:
  
  • Alterations in membrane permeability → cell death
• Genetic deletion:
  
  • Sarcoglycanopathies → limb-girdle muscular dystrophy

Question 15

Composition and function of Vinculin-Talin-Integrin complex

Answer 15

• Integrin, vinculin & talin
• Binds cell membranes
• Co-localises with DGC at costamere, force transducer

Question 16

Characteris, Functions and mutations of Integrin

Answer 16

• Heterodimer (α & β)
• α7B & β1D most common
• Function:
  
  • Mediate cell adhersion & migration
  • Regulate intracellular organisation of actin
  • Signalling
• Mutations:
  
  • Α7 integrin → early onset congenital muscular dystrophy

Question 17

Characteris, functions, and mutations of talin

Answer 17

• Head & rod domain
• 2 isoforms: talin 1 & 2 (↑expression)
  
  • Both essential for muscular development
• Function:
  
  • Interacts with actin & β integrin
  • Talin 1 → signals (binds FAK & vinculin),
    
    • Also binds actin → links integrin & cytoskeleton
• Mutation: none in humans
  
  • In mice:
    
    • Loss of talin 1 → progressive myopathy caused by failure of MTJ (myotendinous junction)
    • Loss of talin 2 → viable & fertile, but myopathy associated with defect in MTJ maintenance
    • Lose both → servere myoblast fusion & sarcomere assembly

Question 18

Characteristic, Function, and mutation of Vinculin

Answer 18

• 4 Vh domains, linker domain, Vt domain
• Function:
  
  • Links integrin to actin
• Mutations:
  
  • Spliced variant: Metavinculin
  • Mutation in variant → idiopathic dilated cardiomyopathy

Question 19

What is the purpose of having 2 complexes?

Answer 19

Both complex have lapping, thus also compensatory functions, like a back-up system

Question 20

Characteristic, Function, Mutations of Desmin

Answer 20

• Muscle specific type 3 intermediate filament protein
• Expressed in skeletal, smooth, cardiac muscles
• Not required for normal muscle development
• But required for maintenance of myofibril integrity
• Function:
  
  • Links adjacent myofibres at Z-line to myofibrils at costamere
• Mutation:
  
  • Desminopathy
  • Gene prevents formation of desmin → misalgin sarcomere → cell death
    
    • Very rare

Question 21

Characteristic, Function, Mutation of Plectin

Answer 21

• Expressed in striated muscles
• 12 isoforms (1, 1b, 1d, 1f most expressed)
• 1&1f → localsied to costomere
• 1d → associated with Z discs
• 1b → localised to mitochondria
• All allows desmin to be localised correctly
• Function:
  
  • Localises desmin
  • Links actin, microtubules & intermediate filaments
• Mutation:
  
  • Epidermolysis bullosa simplex (EBS)
  • Blistering of skin

Question 22

Characteristic, Function, Mutations of Titin

Answer 22

• Largest protein
• Anchors at Z & M lines
• Rigid contact with thick filament along A band
• Flexible along I band
• Protects muscle fibres from overstretching
• Function:
  
  • Framework
  • Centres thick filament during contraction
  • Acts as spring
  • Signalling
• Disease:
  
  • Titinopathy → Limb-girdle muscular dystrophy
  • Disrupts titin’s interactions → muscle weakness & wasting
  • Weakness mainly confined to lower limbs