Evidence for multisite ADP-ribosylation of neuronal phosphoprotein B-50/GAP-43

The neuronal phosphoprotein B-50/GAP-43 is associated with neuronal growth and regeneration and is involved in the calcium/CaM and G_o signal transduction systems. In particular, B-50 interacts uniquely with CaM by binding in the absence of Ca²⁺. Previously identified as a major neuronal substrate for protein kinase C, which releases CaM via phosphorylation, B-50 has more recently been shown to be a substrate for endogenous ADP-ribosyltransferases. In the present study, we utilized amino acid modification with iodoacetamide and chemical stability to mercury and neutral hydroxylamine to demonstrate that the predominant site of ADP-ribosylation is Cys 3 and/or Cys 4. Chymotryptic peptide mapping further revealed a second, less labelled site of ribosylation in the C-terminal region. The results also demonstrate that, in contrast to PKC phosphorylation, ADP-ribosylation of B-50 does not mediate CaM binding. Since Cys 3 and Cys 4, by palmitoylation, are important for membrane anchoring, our findings suggest that ADP-ribosylation of B-50 may have a role in directing the intracellular localization of the protein. Hence, ribosylation of B-50 may mediate where B-50 interacts with signal transduction pathways.