ADP‐Ribosylation of Membrane Proteins in Cholinergic Nerve Terminals

Abstract: Lysed Torpedo synaptosomes or washed synaptosomal membranes were incubated with [³²P]NAD⁺ and subjected to electrophoresis on SDS‐polyacrylamide gels. More than eight membrane proteins were ADP‐ribosylated. The most intensely labeled proteins were those of M_r= 62,000 and 82,000. Radiolabeling was more intense in synaptosomes than in other subcel‐lular fractions. Cholera toxin caused ribosylation of additional synaptosomal proteins with M_r= 42,000 and (in some preparations) 49,000. Neither endogenous nor cholera toxin‐catalyzed ADP‐ribosylation required added guanyl nu‐cleotides. Cholera toxin increased the adenylate cyclase activity of synaptosomal membranes, suggesting that the cholera toxin substrates are regulatory components of adenylate cyclase in these synaptosomes.