Distribution of six synaptic membrane antigens in subcellular fractions of rat brain cortex

The subcellular distribution in rat brain cortex of six synaptic membrane antigens (56K, 58K, 62K, 63K, 64K, 66K) was studied by rocket immunoelectrophoresis, using antiserum to a highly purified synaptic plasma membrane fraction. Initial analysis of the insoluble portion of subcellular fractions showed that these antigens were also present in smooth microsomes, rough microsomes, and synaptic vesicles; that only traces were present in synaptic junctions; and that none was present in nuclei, mitochondria, and myelin. A trace amount of activity was also present in synaptic vesicle cytosol, but none in whole brain cytosol. Quantitative measurements of synaptic plasma membranes, smooth microsomes, and synaptic vesicles showed that all six antigens were present in synaptic plasma membranes and smooth microsomes, but that the 66K antigen was absent from synaptic vesicles. The 56K, 58K, 62, 63K, and 64K antigens were present in highest concentration in synaptic plasma membranes, whereas the 66K antigen content was highest in smooth microsomes. Only the 58K, 62K, and 63K antigen were detectable in the membrane fraction of whole brain. Their enrichments in synaptic plasma membranes were 10.9, 5.4, and 5.9, respectively. We conclude that the 56K, 58K, 62K, 63K and 64K antigens are primary components of synaptic plasma membranes. The presence of synaptic plasma membrane antigens in smooth microsomes and synaptic vesicles probably represents material being actively transported, consistent with the hypothesis that proteins of synaptic plasma membranes and synaptic vesicles are hypothesis that proteins of synaptic plasma membranes and synaptic vesicles are transported via smooth endoplasmic reticulum.