Chapter 13 - The Cytoskeleton and Cell Movement

three main types of cytoskeleton filaments

actin filaments, microtubules, intermediate filaments

regulate assembly of actin filaments, cross-linking, and associations with other cell structures

actin-binding proteins

1st step of actin polymerization: a trimer is formed, and monomers are added to either end

nucleation

number of actin genes in mammals

6

number of actin genes in mammal muscle cells

4

number of actin genes in mammal non-muscle cells

2

monomers are ___ at the minus end of actin filaments

removed

monomers are ___ at the plus end of actin filaments

added

the plus end of an actin filament grows 5-10 times faster than the minus end

treadmilling

critical in regulating actin filaments within the cell

function of treadmilling

type of actin that associates with the plus end

ATP-actin

type of actin that disassociates from the minus end

ADP-actin

the point at which ATP is hydrolyzed to ADP in an actin filament

after being added to the plus end, before moving to the minus end

binds ATP-actin and form unbranched actin filaments

formin

binds actin monomers and stimulate exchange of bound ADP for ATP, increasing local concentration of ATP-actin

profilin

initiates growth of branched actin filaments (important in driving cell movement at the plasma membrane)

Arp2/3

stabilizes actin filaments by binding lengthwise along the groove of the filament

tropomyosin

stabilizes actin by binding to the barbed or pointed ends

capping protein

cleaves and degrades actin filaments into small pieces to generate new ends

cofilin

filaments are cross-linked into closely packed parallel arrays

actin bundles

filaments are cross-linked in arrays that form 3-D meshworks with the properties of semisolid gels

actin networks

brings 2 fibers together

cross-linking protein

transmembrane proteins that bind fibroblasts to attach them to the matrix

integrins

cell-cell contacts in epithelial cells that form a continuous adhesion belt around each cell

adherens junctions

transmembrane proteins that help to anchor actin filaments to the plasma membrane

cadherins

fingerlike extensions that increase surface area for absorption

microvilli

proteins that cross-link actin filaments in microvilli

fimbrin and villin

extensions of moderate width, responsible for phagocytosis and amoeba movement

pseudopodia

broad, sheet-like extensions at the leading edge of fibroblasts

lamellipodia

thin projections of the plasma membrane supported by actin bundles

filopodia

regulates actin by initiating either elongation or shortening

Rho proteins

molecular motor that converts chemical energy (ATP) to mechanical energy to generate force and movement

myosin

bundles of thick myosin filaments and thin actin filaments

myofibrils

large cells formed by fusion of many cells during development

muscle fibers

chain of contractile units beginning and ending at Z discs

sarcomeres

H-zone change during muscle contraction

decreases

I-band change during muscle contraction

decreases

A-band change during muscle contraction

no change

during contraction, the sarcomere shortens, bringing Z discs closer together

sliding filament model

type of muscle filament with 2 heavy chains, 2 pairs of light chains, a globular head region and a long α-helical tail

Myosin II

action that releases P_i during contraction

the myosin head binds to a new position on the actin filament

action that releases ADP during contraction

the myosin head returns to its original conformation, which drives actin filament sliding

released ion that triggers skeletal muscle contraction

Ca²⁺

tropomyosin-troponin complex blocks binding of myosin to actin

absence of Ca²⁺

troponin C shifts to expose the myosin binding site on actin, allowing contraction to proceed

presence of Ca²⁺

maintains cell morphology during cell movement

purpose of contractile assemblies in non-muscle cells

division of a cell following mitosis

cytokinesis

assembled by membrane-bound myosin just beneath the plasma membrane

contractile ring of actin and myosin II

pinches the dividing cell in two

function of the contractile ring