Affinity-purified anti-protein I antibody. Specific inhibitor of phosphorylation of protein I, a synaptic protein

Protein I is a synaptic protein which serves as an endogenous substrate for cyclic AMP- and calcium-dependent protein kinases. Antibodies raised against purified Protein I have been isolated from rabbit antiserum by affinity chromatography on Protein I-conjugated agarose column. The purified antibodies were identified as immunoglobulin G by sodium dodecyl sulfate polyacrylamide gel electrophoresis and Ouchterlony double immunodiffusion precipitation test. The purified antibodies not only inhibited the phosphorylation of purified Protein I by exogenous cyclic AMP-dependent protein kinase, but also inhibited specifically the phosphorylation of Protein I by endogenous cyclic AMP-dependent protein kinase in a synaptic vesicle fraction, synaptic junctional complex fraction, synaptic membrane fraction, and crude homogenate of cerebrum. The purified anti-Protein I antibodies also inhibited with a similar potency calcium-dependent phosphorylation of Protein I without affecting the phosphorylation of other proteins. The Fab(t) fragment of anti-Protein I immunoglobulin G, which was produced by tryptic digestion, retained the ability to inhibit specifically the phosphorylation of Protein I. This substrate-directed, specific inhibitor of the phosphorylation of Protein I may provide a unique probe for investigating the function of Protein I phosphorylation.