Putative 51,000-Mr protein marker for postsynaptic densities is virtually absent in cerebellum.

Abstract

Rat cerebrum and cerebellum contain numerous asymmetric synapses characterized by the presence of a postsynaptic thickening prominently stained by phosphotungstic acid and other electron-dense stains suitable for EM. A 51,000-MW protein, copurified in postsynaptic density-enriched fractions from cerebrum, is considered to be a well established marker for the postsynaptic density. On the basis of 2 criteria, the studies demonstrate that the 51,000-MW protein marker for postsynaptic densities is virtually absent in cerebellum. First, it is present in negligible amounts in deoxycholate-insoluble fractions from cerebellum, but abundant in parallel fractions from cerebrum. Secondly, the 51,000-MW protein, which binds 125I-calmodulin after SDS PAGE [sodium dodecyl sulfate polyacrylamide gel electrophoresis], is readily visualized in membrane samples from cerebrum but is virtually undetectable in cerebellar samples. Reexamination of the role of the 51,000-MW protein in postsynaptic density structures is required.