Presynaptic localization of phosphoprotein B-50.

A major phosphoprotein of synaptic membranes, the phosphorylation of which is stimulated by Ca2+ and inhibited by ACTh, appears to be identical with protein B-50 described by Zwiers, Schotman and Gispen [40]. We have investigated its subsynaptic localization by means of a variety of subfractionation techniques and compared it with that of a number of other phosphoproteins found in synaptic membranes. It appears to be predominantly, if not exclusively, associated with presynaptic membranes of low bouyant density. This localization pattern is similar to, but somewhat more extreme than that exhibited by Protein I, as a brain specific phosphoprotein studied by Greengard and his collaborators [11].