Myosin is a motor that runs along (or tugs at) actin.

Actin-based movement of vesicles

Chromosome migration

Myosin-driven cell shape changes

Muscle contraction

All myosins have head(s), neck andtails

13 family members in humansBut know Myosin I, II, and V

Head has ATPase and actin bindingdomains, actin accelerates ATPase

Neck regulates the head, lightchains bind calcium

Tails have specific binding sites

Gentle digestion releases domains

Myosin II

Experimental tools for your toolbox:

Sliding filament assay, Optical trap

ATP requirement, conditions, direction, rates, forcemovement in discrete steps (cog wheel)

Know the contraction cycle!!

1. No ATP- rigor state2. ATP causes release and movement3. ATP hydrolysis allows rebinding4. Loss of Pi causes power stroke5. ADP release restores to rigor position

The myosin-actin interaction drives muscle contraction

Muscles have the power of a car on a per mass basis

Skeletal, isotonic contractions

Smooth, isometric contraction

Tropomyosin, ropelike proteinbinds actin.

Troponin, binds TM and calciumCalcium binding nudges TM

TM movement exposes myosin site on actin

With myosin able to bind, thecontraction cycle proceeds

Smooth and skeletal contractions differ

Instead of troponin, smooth muscles use caldesmon at low calciumCalcium levels change more slowlyThe network is less ordered.

Caldesmon binds

Caldesmon off by CaM

Additional control by kinases

It’s also possible to control contraction by myosin modification

Examples:1. Invertebrate muscle, calcium binding to myosin LC2. Vertebrate smooth muscle, phosphorylation of myosin LC by MLCK in a Ca++/CM cycle3. Or by Rho kinase

These pathways are independent of the action potential. They represent other signaling pathways.