Structure of contractile proteins

Structure of contractile proteins

Contraction depends on two contractile proteins organised into the thick and thin myofilaments of the sarcomere, plus two regulatory proteins that control when contraction can occur.

Thick filaments are bundles of the protein myosin. Each myosin molecule is a monomer of two intertwined meromyosin chains, and each molecule has two structural parts: a globular head with a short arm, and a long tail. The short-arm portion is called heavy meromyosin (HMM); the tail portion is the light meromyosin (LMM). The head carries an actin-binding site, an ATP-binding site, and an ATPase enzyme that hydrolyses ATP to release the energy driving contraction. Myosin molecules bundle together with their heads projecting outward at opposite ends of the thick filament.

Thin filaments are built from two interwound strands of actin. Actin's basic subunit is a small globular protein, G-actin; many G-actin monomers polymerise into a filamentous strand, F-actin, and two F-actin strands twist around each other to form the complete thin filament, which also carries the myosin-binding active sites. Wound along this actin double-helix are two regulatory proteins:

• Tropomyosin, a thread-like protein lying in the groove of the actin helix that, at rest, physically covers the myosin-binding sites on actin, preventing contraction. …