Contractile Proteins Flashcards
What do the structural, spatial, and mechanical functions of cells depend on?
Cytoskeleton
Allows cells to have:
- organization in space
- mechanical interaction with each other and their environment
- ability to rearrange internal components in response to growth, division, and dynamic adaptation to changing situations
- change shape in response to stimuli
- migrate from place to place
- have polarity
- remain in close contact with each other and form stable sheets/layers
Cytoskeleton
Determine shape of cell, cell locomotion, and pinching of one cell into 2
Actin and actin-binding proteins
Molecular machines that convert biochemical energy from ATP hydrolysis to mechanical energy that moves organelles along filaments or move filaments on proteins
Myosin (motor proteins)
Actin subunit
G actin (globular actin)
- 375 AA polypeptide carrying tightly associated ATP or ADP
- small in size & diffuse rapidly into cytoskeleton
- assembled/polymerized head to tail to form tight, right handed helix
G-actin
Right-handed G-actin helix
F actin (filamentous actin)
Polarity of F-actin
- slow growing minus end
- fast growing plus end
Importance of polarity of actin filaments
- assembly
- establishing unique direction of myosin movement relative to actin
Important mechanism by which cells control shape and movement
Regulation of actin filament formation
- associate spontaneously
- unstable
- disassembly readily
Small oil Gomes’s
For new actin filaments to form, what must happen?
Filament nucleation
Filament nucleation
Subunits must assemble into an initial aggregate (nucleus made of 3 actin monomers) that is stabilized by multiple subunit-subunit contacts then elongate rapidly by addition of more subunits
How do actin filaments grow?
Reversible addition of monomers to both ends
Which end elongates faster?
Plus end elongates 5-10 x faster
Actin monomers have ___ which is hydrolyzed to _____ following filament assembly.
- bound ATP
- hydrolyzed to ADP and Pi
Compare actin monomers to which ATP is bound vs. ones with bound ADP
- ATP not required for polymerization
- Actin monomers to which ATP is bound polymerize more rapidly
Plays a key role in assembly and dynamic behavior of actin filaments
ATP binding and hydrolysis
Since actin polymerization is reversible, filaments can ____ as needed by the _____.
- Depolymerize
2. Dissociation of actin monomers
dependent on concentration of free monomers
Equilibrium between actin monomers and filaments
Rate at which actin monomers are incorporated into filaments is proportional to what?
Concentration
There is a _____ of actin monomers at which _______. At this concentration, monomers and filaments are in apparent _____.
- Critical concentration
- Rate of their polymerization into filaments equals rate of dissociation
- Equilibrium
The rate of subunit association is proportional to ______.
Concentration of free monomers
The rate of subunit dissociation is independent of _____.
Monomer concentration
K(off) = Cc x k(on)
An apparent equilibrium is reached at critical concentration of monomers (Cc)
- organized into higher order structures, forming bundles of 3D networks within cells
- associated with other cell structures such as the PM
- abundant beneath PM, where they form network
Actin
Functions of actin
- mechanical support
- determines cell shape
- allows movement of cell surface
- enables cells to migrate, engulf particles, and divide
Regulates organization of actin network and functional structures
Actin binding proteins
- helps achieve cross-linking of F-actin
- have at least 2 actin-binding domains
Accessory proteins
Nature of association of filaments is determined by?
Size and shape of the cross linking proteins
Typically small ridged proteins that force filaments to align closely with each other
Bundling proteins
Two types of structures that actin filaments are assembled into
- actin bundles
- actin networks
Actin bundles are cross-linked into _____
Closely-packed parallel arrays
- loosely cross-linked in orthogonal arrays that form 3-D meshwork with more flexible gel-like properties
- makes cells flexible
- changes polarity of actin filament
Actin networks
- parallel bundle
- tight-packing prevents myosin II from entering bundle
Actin filaments and fibrin
- contractile bundle
- loose packing allows myosin II to enter bundle
Actin filaments and alpha-actinin
- made of closely spaced actin filaments in parallel
- fimbrin monomer binds to actin filaments
- has 2 actin-binding domains (ABD)
- holds 2 parallel filaments close together
- filaments have same polarity
- supports projections of PM
- increase cell surface and gives structure stability
- allows placement for additional receptors, channels for signaling ,transport, and uptake of nutrients
Parallel bundles
- loosely bundled actin filaments
- looseness due to structure of cross-linking proteins 9ex: alpha-actinin)
- binds as a diner
- bigger protein = distance between actin bundles
- filaments separated by greater distance
- allows motor protein (myosin) to interact during contraction
- contractile ring used in mitosis: allows squeezing of cytoplasm by contractile ring to cause cell division
Contractile bundles
- actin filaments in networks held together by large actin binding proteins
- filaments
- binds actin as a diner
- actin binding domains located on opposite ends of the diner
- creates a 3D meshwork
- present in cells that need to withstand forces
Actin-bundling proteins
Thin projections of PM supported by actin bundles. Formation and retraction of these structures is based on the regulated assembly and disassembly of actin filaments.
Filopodia
Broad, sheet-like extensions at the leading edge of cell, containing network of actin filaments
Lamellipodia
Based on actin filaments cross-linked into a 3D network, responsible for phagocytosis
Pseudopodia
Spectrin
ABP contained by erythrocytes
- tetramers associate laterally
- form actin network that creates cortical cytoskeleton
- spectrin-actin network interacts with membrane proteins via interactions with ankyrin, protein 4.1
Spectrin
- mutations in erythrocytes cortical cytoskeleton proteins
- hereditary spherocytosis (HS)
- impaired deformability and reduced stability of RBCs
- movement from large vessels to capillaries require flexibility and deformability
- spherical RBCs
- membrane breaks down, RBCs damaged and die
- anemia
- jaundice
- splenomegaly
Hereditary spehrocytosis
- superfamily of motor proteins
- 20 diff myosin types in eukaryotes
- all move along actin filaments via ATP hydrolysis
Myosin
- bipolar filaments
- tails associate to form shaft of filament
- heads exposed at both ends
Skeletal muscle myosin II
3 domains of myosin
- head: contain actin binding and ATP binding sites, has ATPase activity
- neck: flexible region, binds myosin light chain peptides
- tail: intertwine to bring myosin head regions in close proximity, bind membrane/organelles
Membrane association and endocytosis
Myosin I
Contraction
Myosin II
Organelle transport
Myosin V
Myosin in the absence of ATP
Myosin attached to actin filament
Myosin bound to ATP
Conformation change with release of actin
What causes ATP hydrolysis?
Binding of actin
What drives the “power stroke”?
Release of P and I elastic energy > straightens myosin > moves actin filament left
Steps of the power stroke
- ATP binds and the head is released from actin
- Hydrolysis of ATP to ADP and Pi; myosin head rotates into “cocked” state
- Myosin head binds actin filament
- “Power stroke” = release of P and i elastic energy straightens myosin; moves actin filament left
- ADP released, ATP bound, head released from actin
Power stroke (Velocity) mechanism is proportional to _____.
Length of neck domain
- thick filaments = 6 myosin polypeptide chains (1 pair of heavy chains which form the tail and 2 pairs of light chains which form the head)
- thin filaments = 3 proteins = actin, tropomyosin, troponin
Skeletal muscle
Skeletal muscle fiber at rest
- myosin binding sites covered by tropomyosin so actin/myosin cannot interact
- troponin is calcium-binding protein that helps initiate contraction
Unit of skeletal muscle
Sarcomere
Non-muscle cell contractions
- contain several types of actin-myosin structures similar to skeletal muscle fibers (less stable/organized)
- formed in transient manner as needed by cell
- bundles of F-actin and myosin II form contractile ring
- myosin movement along actin filaments creates cleavage furrow
Cytokinesis
____ carries cargo along actin filaments.
Myosin V
- part of protein complex that linked cytoskeleton of muscle fibers to the surrounding CT
- long protein with numerous redundant coils that provides a structural link b/w cytoskeleton of the muscle cell and the ECM
Dystrophin
- acts like shock absorber during contraction
- stabilizes the sarcolemma and prevents contraction-induced injury
Long protein with numerous redundant coils that provides a structural link b/w cytoskeleton of the muscle cell and the ECM
Without functional dystrophin to support muscle strength and stability, muscle fibers are _____.
Easily damaged
Loss of dystrophin
Duchenne’s muscular dystrophy
- X-linked recessive disorder
- abnormal dystrophin gene
- progressive muscle wasting
DMD (Duchenne’s Muscular Dystrophy)
Milder form of DMD
Becker Muscular Dystrophy (BMD)
- connects cytoskeleton to basal lamina
- stabilizes PM
- hundreds of mutations
Dystrophin
- Little to no expression of dystrophin
- OOF mutants
Duchenne Muscular Dystrophy (DMD)
- Smaller protein with partial function of dystrophin
- IF mutation
Becker Muscular Dystrophy (BMD)
Drugs being developed to treat dystrophin
- gene therapy
- dystrophin replacement
- drugs to prevent exon skipping
