Function of a calmodulin in postsynaptic densities. III. Calmodulin-binding proteins of the postsynaptic density.

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Abstract
A method was developed for binding calmodulin, radioiodinated by the lactoperoxidase method, to denaturing gels and was used to attempt to identify the calmodulin-binding proteins of canine cerebral cortex postsynaptic densities (PSD). Calmodulin primarily bound to the major 51,000 Mr [molecular ratio] protein in a saturatable manner; secondarily bound to the 60,000 Mr region, 140,000 Mr region and 230,000 Mr protein; and bound in lesser amounts to a number of other proteins. The major 51,000 Mr calmodulin-binding protein is one of unknown identity. Binding of iodinated calmodulin to these proteins was blocked by EDTA, EGTA [ethyleneglycol-bis(.beta.-aminoethyl ether)N,N,N'',N''-tetraacetic acid], chlorpromazine and preincubation with unlabeled calmodulin. Calmodulin iodinated by the chloramine-T method, which inactivates calmodulin, did not bind to the PSD but bound nonspecifically to histone. Calmodulin did not bind to proteins from a variety of sources for which calmodulin interactions were not found. Except for 3 proteins, all of the proteins of synaptic membranes that bind calmodulin could be accounted for by proteins of the PSD which are a part of the synaptic membrane fraction. The major 51,000 Mr protein and the corresponding iodinated calmodulin binding were greatly reduced in cerebellar PSD and this difference between cerebral cortex and cerebellar PSD is discussed in light of the possible function of calmodulin in synaptic excitatory responses.

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