Microfilament based motility

myosins

ATP-dependent motors

Myosin function

interacts with and exerts force on actin microfilaments

myosins are generally ____ end directed motors

plus

4 cellular events that myosin functions in

1. muscle contraction

2. cell movement

3. phagocytosis

4. vesicle transport

Myosin structure

Has atleast one heavy chain, with a globular head group attached to a tail of varying length and a regulatory light chain

What part of myosin binds actin?

the globular domain

How does myosin move along an actin filament?

uses ATP hydrolysis for energy to move along the actin filament

Myosin light chains

play a role in regulating the ATPase (needed for movement)

Type II Myosins structure

2 heavy chains, each with a globular head, a hinge region, and a rod-like tail

Type 2 myosin light chains

1 essential light chain and 1 regulatory light chain associated with each heavy chain

basic function of type 2 myosin

pull arrays of actin filaments together, resulting in contraction of a cell or group of cells

thick filament is made up of

hundreds of type II myosin molecules bunched together in a staggered formation

Each muscle fiber contains numerous

myofibrils

myofibrils are divided into repeating units called

sarcomeres

each sarcomere contains bundles of

thin and thick filaments

thin filaments contain

actin, troponin, and tropomyosin

thick filaments contain

myosin

Dark bands in skeletal muscle are

A bands

Light bands in skeletal muscle are

I bands

In the middle of each I band is a

dense Z line

What defines a sarcomere?

the distance from one Z line to another

What end of actin microfilaments are anchored to the Z line?

plus ends

In thick filament, how is myosin oriented?

in opposite directions in the two halves of the filament (staggered)

How is myosin connected to adjacent actin filaments in order to form cross bridges?

protruding heads of myosin

Protein composition of thin filaments

F-actin, intertwined with tropomyosin and troponin

troponin

a calcium sensitive switch that regulates myosins access to the actin microfilament

What model explains muscle contraction?

The sliding filament model

The sliding filament model

muscle contraction is due to thin filaments sliding past thick filaments, with no change in the length of either

The force generated during muscle contraction depends on

the number of actin-binding domains that make contact with the thin filaments

muscle contraction

myosin 2 mmoves towards the plus ends so that the thick filaments move towards the Z lines during contraction

___ holds filaments together and ___ powers their movement

cross-bridges; ATP

Cross-bridges

regions of overlap between thin and thick filaments

Cross bridges are formed between

F-actin of thin filaments and myosin heads of thick filaments

What mediates cross bridge formation

structural change in troponin

The regulation of muscle contraction depends on

calcium

Which regulatory proteins regulated the availability of myosin binding sites on actin filaments and in what manner do they act?

tropomyosin and troponin in a calcium dependent manner

What blocks myosin binding sites on actin?

tropomyosin

What must be moved for cross bridges to form?

tropomyosin on myosin binding sites

What happens when calcium concentration is low?

tropomyosin clocks the myosin-binding sites on the actin filaments, preventing interaction

What happens at higher concentration of calcium

calcium binds TnC, causing tropomyosin to shift and allowing myosin to bind

Nerve impulses from ___ activate the appropriate muscle cells

motor neurons

What happens when nerve impulses to the muscle cease

calcium levels decline quickly, and the muscle relaxes

Acetylcholine

opens Na+ channels on muscle cell-depolarization

Depolarization

opens voltage gated Ca2+ channels

Ca2+ stimulates ___ in the ____

ryanodine receprots; sarcoplasmic reticulum

Ca2+ decline in the sacroplasm is due to

ATP dependent Ca2+ pumps in the sacroplasmic reticulum

Smooth muscle is more similar to

non muscle cells than to skeletal muscle

In comparison to skeletal muscle contractions, smooth muscle contractions are

relatively slow and of greater duration

Structure of smooth muscle

long and thin cells with pointed ends; no striations

instead of Z lines, smooth muscles have

dense bodies, plaque like structures

In smooth muscle, where are actin and myosin filaments anchored?

at both ends to the dense bodies

What is the first thing that happens one calcium concentration increases in smooth muscle cells?

Ca2+ binds to and activates Calmodulin

In the smooth muscle contraction cascade, what happens once calmodulin is activated?

it activates myosin light chain kinase (MLCK), which phosphorylates a regulatory light chain of myosin

In the smooth muscle contraction cascade, what happens once myosin light chain phosphorylation has occured?

a conformation change in myosin occurs, promoting its filament assembly into filaments

myosin is also activated so that it can interact with actin filaments to undergo cross bridge formation

What happens after contraction in smooth muscle?

MLCK is inactivated

What removes phosphate from the myosin light chain?

Myosin light-chain phosphatase

what happens once phosphate is removed from the myosin light chain?

the muscle cell relaxes