Actin and Myosin

Padrick

actin is highly [blanked] in eukaryotes

conserved

[blank] actins are >98% made up of skeletal, smooth, and cardiac actins

muscle

[blank] actins are >98% made up of beta and gamma actins

cytoplasmic

Actin monomer has an unusual subdomain structure that binds [blank] and hydrolyzes very slowly

ATP

actin monomers combine into polarized filaments that hydrolyze much more quickly than their [blanks]

monomers

[blank] is slow and highly concentration dependent

nucleation

[blank] is fast with the asymmetric or “plus” end being faster than the pointed or “minus” end

elongation

[Actin]eq is a [blanked] value and is the ‘critical concentration’

fixed

at cellular concentrations actin subunits are more stable in filament than free in solution, so [blank] can be used to do mechanical work

polymerization

[blank] is the mechanism of actin doing mechanical work

Brownian Ratchet

little free actin exists in cells, so actin binds to [blank] which buffers the free concentration and prevents elongation

b-Thymosin

[blank] is the actin nucleotide exchange factor and most actin is bound to it

profilin

profilin allows [blank] end addition where most of elongation occurs

barbed

almost all cellular actin monomers are bound to profilin or b-thymosin, thus [blank] is prevented in cells

nucleation

nucleation factors allow the cell to [blank] new filaments at the correct place and time

initiate

[blank] proteins prevent elongation and depolymerization at filament ends

capping

elongation factors—[blank] and ENA/VASP proteins—control actin delivery and are ‘anti-capping proteins’

formins

ATP [blank] de-stabilizes the filament and severing factors create more pointed ends

hydrolysis

[blank] pointed ends depolymerize under typical cellular conditions

uncapped

[blank] are broad, flat, rapidly polymerizing protrusions seen in cells in 2D environments

lamellipodia

lamellipodia are rich in densely [blanked] actin

branched

actin filaments are nucleated at the membrane by Arp2/3 complex and are activated by [blank] proteins

WASp/Scar

aging filaments are enriched in ADP actin, which are targeted for recycling by [blank]

cofilin

[blank] actin is bound by profilin, recharged with ATP and ready for recycling into new filaments at the leading edge

depolymerized

[blank] are fast growing parallel bundles of actin

filopodia

different bundling proteins result in different structures, and alpha-actin is an [blank] bundling factor

anti-parallel

[blank] is an actin filament binding protein that uses the energy from ATP hydrolysis to ‘walk’ toward the barbed end of the filament

myosin

[blank] muscle is sometimes called ‘striated muscle’ due to the appearance of regular light and dark bands upon imaging by electron microscopy

skeletal

the repeating element in skeletal muscle is called the [blank]

sarcomere

in skeletal muscle, unregulated contraction is prevented by [blank] positioning Tropomyosin in the way of the Myosin binding site on actin

troponin

Upon [blank] stimulation, Troponin bind calcium ions, and is released from actin filament binding, allowing contraction to proceed

Ca2+

in smooth muscle, unregulated contraction is prevented by [blank] inactivity of myosin, due to an unphosphorylated light chain

basal

Upon Ca2+ stimulation, [blank] binds Myosin Light Chain Kinase (MLCK), activating it. Light Chain is phosphorylated, allowing myosin to bind actin, and contraction to proceed

calmodulin

when a nerve impulse is received at the myofibril, it spreads over the plasma membrane, and into membranous folds called [blanks]

T tubules

T tubules are closely associated with a membrane enclosed organelle called the [blank] reticulum

sarcoplasmic

an action potential in the T tubule causes calcium release from the sarcoplasmic reticulum, triggering [blank]

contraction