BIOS 3450 - Microfilaments

_____ is a highly conserved and abundant eukaryotic cell protein.

actin

Cells assemble diverse structures of _____ _____ for different functions.

actin filaments

G-actin reversibly assembles into polarized _____ filaments.

F-actin

What are F-actin filaments composed of?

two protofilaments, in which the actin subunits are all oriented in the same direction

_____ are wound around each other to form a helix with the actin nucleotide-binding site exposed on the _____ end of each _____

protofilaments, negative, protofilament

The _____ gives the cell shape, structure, and motility.

cytoskeleton

What are the three components of the cytoskeleton?

microfilaments, microtubules, intermediate filaments

What are microfilaments made of?

actin protein, which is highly conserved among species

Actin filaments have _____.

polarity

Actin addition happens at the ____ end, and actin removal happens at the ____ end

+, -

What are the three phases of in vitro G-actin polymerization?

nucleation, elongation, steady state

What is actin treadmilling?

actin is added to the + end and lost from the - end

Actin binding proteins facilitate ____ or _____

polymerization or depolymerization

What is the name of the cycle for polymerization?

profilin cycle

What is the name of the cycle for depolymerization?

coflin cycle

Thymosin-beta4 binds ATP-G-actin and 'holds' it by preventing it from _____.

polymerizing

_____ _____, such as CapZ or Tropomodulin bind the + or - ends of actin filaments and prevent further polymerization or depolymerization, respectively.

capping proteins

Functionally different actin-based structures are nucleated by _____ and _____ complexes.

formins, Arp2/3

What are two functions of Arp2/3-dependent actin polymerization?

moves pathogenic bacteria and endocytic vesicles within cells and pushes the leading edge membrane forward in moving cells

Actin nucleation by the _____ protein FH2 domain promotes growth of long, linear strands.

formin

What do different classes of formin proteins do?

nucleate actin assembly in different ways

What is responsible for assembly of long actin filaments found in muscle cells and stress fibers?

formins

Actin nucleation by the Arp2/3 complex leads to _____ _____ _____.

branched actin structures

What does Listeria use actin polymerization for?

intracellular movement

What powers the movement of the vesicle?

ARP 2/3-dependent actin assembly during endocytosis

Removal of pathogens by _____ _____ is driven by actin dynamics.

cell phagocytosis

_____ _____ _____ are coated with antibodies leading to exposure of Fc domain.

pathogenic invading bacteria

What recognizes the Fc domain of the antibody?

Fc receptors on a leukocyte cell

What does the interaction between the Fc domain of the antibody and Fc receptors on the leukocyte cell induce?

assembly of microfilaments that help provide the force to engulf the bacteria

What happens to the bacteria inside a leukocyte?

it is killed

_____ _____ are attached laterally and end-on to membranes.

actin filaments

Actin _____ proteins mediate the formation of diverse actin structures.

cross-linking

Actin cross-linking proteins mold _____ into different structures.

F-actin

How many domains does each protein have to promote the formation of distinct structures in different cells or cellular locations?

two

_____ and _____ help link actin to the plasma membrane.

Ezrin and dystrophin

_____ _____ converts ATP hydrolysis energy to mechanical work on actin filaments.

crossbridge cycle

What are actin-based motor proteins called?

myosins

What does myosin II do?

organizes into bipolar filaments

What are the four components of Myosin II?

heavy chains, light chains, regulatory chains, two actin binding sites

What is the function of myosin class I?

membrane association, endocytosis

What is the function of myosin class II?

contraction

What is the function of myosin class V?

organelle transport

What are the three common classes of myosin?

Class I, Class II, Class V

Muscle cells are typically _____ and consist of _____, which are composed of _____.

cylindrical, myofibrils, sarcomeres

What are the thick filaments of sarcomeres composed of?

myosin

What are the thin filaments of sarcomeres composed of?

actin

When do myosin heads walk toward the + end of the thin actin filaments?

in the presence of ATP and Ca2+

Where are thin actin filaments anchored?

Z disc

How does the movement of myosin cause the sarcomere to contract?

by pulling filaments together

Ca2+ binding to _____ and _____ causes them to expose actin binding sites for myosin.

troponin, tropomyosin

What is the process for myosin light-chain phosphorylation regulating smooth muscle contraction?

Ca2+ binds calmodulin (CaM) -> activates myosin light chain (MLC) kinase -> phosphorylates the myosin light chain -> myosin forms contractile polar filaments

What do Rho GTPase family proteins do?

regulate formation of different actin filament organizations and myosin II activity to direct cell motility

What is cell locomotion?

a wide variety of cells move within the body to arrive at destinations appropriate for their function

How are changes induced by signals in the actin cytoskeleton?

signals interact with receptors -> leads to activation of GTP-binding proteins -> these proteins interact with effector proteins -> actin/myosin activity

What is the process of ATP-driven myosin II movement along actin filaments?

myosin releases actin on binding ATP -> ATP is hydrolyzed -> myosin head moves -> myosin contacts actin -> connects with actin and moves it -> Pi is released

What causes skeletal muscle contraction?

thin filament Ca2+ regulation

What causes smooth/nonmuscle cell contraction?

thick filament Ca2+ regulation