Substrate specificity of the autocatalytic protein that primes glycogen synthesis

The autocatalytic protein that primes muscle-glycogen synthesis, and which glucosylates itself from UDPglucose, is inhibited by maltose. Investigation of the reason for the inhibition led to the finding that the protein will glucosylate substrates other than itself. p-Nitrophenyl alpha-glucoside, alpha-maltoside, alpha-maltotrioside and alpha-maltotetraoside each inhibit self-glucosylation of the protein by acting as alternative acceptor substrates. The alpha-maltoside is the best acceptor. The alpha-maltohexaoside did not act as an acceptor but was an effective inhibitor. These findings help to explain the self-limiting nature of the autocatalytic extension of the maltosaccharide chain of the protein and suggest that protein self-glucosylation may be an intermolecular event. They may also point to the mechanism by which the autocatalytic protein is initially glycosylated.