Attachment of the Synapse-Specific Phosphoprotein Protein I to the Synaptic Membrane: A Possible Role of the Collagenase-Sensitive Region of Protein I

The purified synapse-specific phosphoprotein Protein I was previously shown to be degraded by a bacterial collagenase, through a series of intermediates, to a collagenase-resistant fragment of molecular weight about 48,000 containing a phosphorylated serine residue. In this study, a purified synaptic membrane fraction containing Protein I was treated with Cl. histolyticum collagenase; membrane-bound and membrane-free proteins were then phosphorylated using gamma-32P]ATP and analyzed by SDS-polyacrylamide gel electrophoresis and autoradiography. It was observed that Protein I bound to the synaptic membrane was susceptible to the collagenase and degraded to fragments of molecular weights about 68,000, 62,000, and 48,000; the 68,000 fragment remained bound to the membrane whereas the 62,000 and 48,000 fragments were dissociated from the membrane. These observations suggest that the peptide moiety of mol. wt. 6000, present in the 68,000 fragment but absent from the 62,000 fragment, may play a crucial role in anchoring Protein I to the synaptic membrane.