Myosin light chain definition

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The light chains of the muscle protein myosin. Each molecule of myosin is composed of two heavy chains and two pairs of light chains. The light chains have a molecular weight of about 20 kD and there is one dissimilar pair of light chains associated with each heavy chain.

The proteins all have sequence homology to calmodulin, but not all with calcium binding activity.

Several types are known: regulatory light chains (LC 2, DNTB light chains) probably regulate the ATPase activity of the heavy chain directly (through the binding of calcium) or indirectly (activating when they themselves are phosphorylated by myosin light chain kinase) and essential light chains (LC 1, LC 3, alkali light chains), which have a more subtle and apparently nonessential role.

In molluscan muscle the EDTA light chains (similar to LC 2 from vertebrate muscle) confer calcium sensitivity on the myosin itself.

The light chains are "calmodulin-like" proteins that bind calcium. Two of them can be removed easily, and two with difficulty. The light chains bind the heavy chains in the vicinity of the head groups of the myosin.

(12 Dec 1998)