Actin filaments are polar cytoskeletal polymers assembled from many copies of the protein actin. Individual actin subunits bind ATP, and cells regulate where filaments nucleate, grow, shrink, branch and bundle.

Because actin subunits are oriented consistently, an actin filament has two structurally distinct ends. This polarity allows the filament to grow differently at its two ends and provides directional tracks for motor proteins.

Actin networks can push or resist cell boundaries

When actin polymerizes next to a membrane, addition of new subunits can generate force against that membrane. Cells use regulated actin assembly to change shape, form protrusions and reorganize their cortex, the actin-rich layer beneath the plasma membrane.

Actin filaments can also be cross-linked into networks or bundles that bear tension.

Myosin converts chemical energy into force along actin

Myosins are actin-binding motor proteins. A myosin motor undergoes an ATP-driven cycle of binding, force generation and release from actin.

A simplified mechanical cycle is

myosin binds actin
      ↓
ATP-dependent conformational cycle
      ↓
myosin changes position and exerts force
      ↓
release and repeat

When many myosin motors interact with oppositely oriented actin filaments, their local motions can create contraction of an actin network rather than merely transport one object along one track.

Such an actomyosin contractile system underlies processes including animal-cell cytokinesis, cell migration and muscle contraction.

Polymerization and motor activity are different sources of force

Actin systems can therefore generate motion in at least two conceptually distinct ways:

• polymerization forces, produced as filaments grow against a boundary;
• myosin-generated contractile forces, produced as ATP-powered motors pull on actin filaments.

These mechanisms can cooperate in one cell, but they should not be conflated.

The reusable idea is that actin is a dynamic, polar structural polymer, while myosin is an ATP-powered motor that converts chemical free energy into directed mechanical force on the actin network.