32: Contractile Proteins Flashcards
Dystrophin function
Links muscle fiber cytoskeleton + ECM/basal lamina; acts as a shock absorber for contraction, stabilizes sarcolemma
Dystrophin structure
Super big; 427 kDa (largest human gene, 79 Exons), has numerous redundant coils
What are 2/3rds of all dystrophin mutations
Large internal deletions
Out of frame vs in-frame dystrophin mutation
Out of frame -> frameshift -> DMD
In frame -> BMD
Genetics of DMD
X-linked recessive, affecting 1 in 3,500 boys
A few treatment options for DMD
Gene therapy, dystrophin replacement, drugs to prevent exon skipping
G actin structure
375AA polypeptide divided into two lobes, with an ADP or ATP bound to the bottom of the cleft, connecting the lobes
F actin
Tight right-handed helix formed by two strands of G actin polymerizing
Why is polarity of actin important?
For determining which way myosin moves along it
Three steps of actin filament assembly
- Nucleation
- Elongation phase
- Steady state
Nucleation of actin
3 G-actin monomers aggregate into an oligomer
ATP hydrolysis in actin formation
ATP is bound to monomers -> hydrolyzed to ADP + Pi following filament assembly
Where are actin filaments particularly abundant in the cell?
Beneath plasma membrane
A requirement for actin binding proteins
Must have at least two actin-binding domains (ABDs)
Actin bundles vs actin networks
Actin bundles: cross-linked into parallel arrays (filament polarity aligned)
Actin networks: loosely cross-linked into orthogonal arrays (polarity doesn’t align)
Two types of actin bundles and the protein that holds them together
- Parallel bundles: fimbrin
2. Contractile bundles: a-actinin
Parallel vs contractile bundles of actin: packing
Parallel bundles: tight packing -> prevents myosin II from entering
Contractile bundles: loose packing, allowing myosin II to enter for contraction
Parallel bundle function
Supports projections of plasma membrane, increases cell surface, structural stability, allows for additional receptors to be placed on plasma membrane
Example of parallel bundles vs contractile bundles
Parallel: microvilli
Contractile: contractile ring in mitosis
Actin network function
Forms 3D meshwork for flexible, gel-like properties
Example of a protein that holds actin networks together
Filamen
Spectrin
An actin binding protein in plasma membrane of RBCs that creates a cortical cytoskeleton
Two other proteins that assist spectrin
Ankyrin, protein 4.1
HS mutation
Spectrin, ankyrin, protein 4.1
What happens in hereditary Spherocytosis?
RBCs are spherical and have impaired deformity, reduced stability, difficult to enter capillaries -> membranes break down and RBCs die -> anemia, jaundice, splenomegaly
Three domains of myosin and their functions
- Head: binds actin and ATP
- Neck: binds myosin light chain
- Tail: intertwine to bring heads closer, bind membranes/organelles
Powerstroke mechanism and neck of myosin
Longer neck -> proportional to powerstroke mechanism movement
What direction on actin do myosin I, II, and V move towards?
Plus end
Myosin I, II, and V structures
I: 1 heavy chain + head + neck
II: 2 heavy chains + 2 heads + 2 necks
V: 2 heavy chains + 6 light chains per neck
Only single-headed myosin
Myosin I
Myosin I function
Some associate directly with membranes through tail-lipid interaction
Only myosin class that can assemble into bipolar filaments through tail interactions
Myosin II
Myosin V function
Tails associate with organelles/cargo -> carry it along actin filament tracks
Four important functions of myosin-powered movements
- Skeletal muscle contraction
- Smooth muscle contraction
- Non-muscle cell contraction
- Cell migration
Titin structure and function
Attaches Z disc -> middle f myosin filament; stabilizes myosin and prevents overstretching, brings thick filament back to baseline after contraction
What do mutations in titin cause?
Cardiomyopathies
Components of thin filaments
Actin, troponin, tropomyosin
Nebulin
Long protein wrapping around actin, determines length of thin filament
What two proteins stabilize ends of actin?
CapZ: stabilizes + end
Tropomodulin: stabilizes - end
What constantly pumps Ca from Cytosol into SR in a resting muscle?
SERCA; SR Ca ATPase
Smooth muscle contraction steps
- Ca levels rise, Ca binds calmodulin
- Calmodulin activates MLCK
- MLCK phosphorylates myosin light chains -> high affinity for actin
- Powerstroke
What happens when Ca levels drop after smooth muscle contraction?
MLCK becomes inactive, MLCP dephosphorylates myosin light chains -> low affinity state for actin -> relaxation
Non muscle cell actin-myosin structures
Can transiently form actin-myosin structures similar to skeletal muscle as needed, but are less stable and less organized
Cytokinesis
Bundles of F-actin and myosin II form a contractile ring -> cleavage furrow
What factor is expressed in fibroblasts for the localization of actin, causing cell migration?
GFP-actin
