Calmodulin Binding Proteins of the Cholinergic Electromotor Synapse: Synaptosomes, Synaptic Vesicles, Receptor-Enriched Membranes, and Cytoskeleton

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Abstract
Calmodulin binding proteins (CBP) were identified using a gel overlay technique for fractions isolated from Torpedo californica electromotor nerve endings. Different fractions possessed characteristic patterns of CBP. Synaptosomes showed 5 major CBP, MW 220,000, 160,000, 125,000, 55,000 and 51,000. Polypeptides of MW 55,000 and 51,000 were found in the cytoplasm and the others are membrane-associated. The Triton X-100-insoluble cytoskeleton of synaptosomes was isolated in the presence or absence of Ca. The major CBP had MW of 19,000, 18,000 and 16,000. In the presence of Ca, no other CBP were seen. In the absence of Ca, a MW 160,000 polypeptide was present in the Triton cytoskeleton. Synaptic vesicles showed CBP of MW 160,000, 25,000 and 20,000. Membrane fragments enriched in acetylcholine [ACh]receptors contained 2 major CBP, MW 160,000 and 125,000, together with a less prominent protein at MW 26,000. A protein of MW similar to that of fodrin was present in synaptosomes and ACh receptor membrane fragments, but only in small amounts relative to the other polypeptides observed. The H and L chains of clathrin-coated vesicles from pig brain did not bind calmodulin, although strong labeling of a MW 47,000 polypeptide was found. Calelectrin does not bind calmodulin. The possible identity of the calmodulin binding proteins is discussed.