INITIATION OF GLYCOGEN-SYNTHESIS IN YEAST - REQUIREMENT OF MULTIPLE TYROSINE RESIDUES FOR FUNCTION OF THE SELF-GLUCOSYLATING GLG PROTEINS IN-VIVO

The self-glucosylating proteins, G1g1p and G1g2p, are required for gly cogen synthesis in Saccharomyces cerevisiae (Cheng, C., Mu., J., Farka s, I., Huang., If,, Goebl M. G., and Roach, P. J. (1995) Mol. Cell. Bi ol. 15, 6632-6640). G1g2p was shown to be associated with carbohydrate in vivo and was released from the high molecular weight glycogen frac tion by treatment with alpha-amylase. In addition, some G1g2p exists a s a protein of M(r) similar to 43,000, whose proportion is increased i n cells lacking glycogen synthase, Unlike the. mammalian counterpart, glycogenin, the yeast GIg proteins appear to require multiple Tyr resi dues for functionality. In G1g2p, mutation of both Tyr(230) and Tyr(23 2) is necessary to suppress self-glucosylation of purified protein in vitro. The mutant protein is still capable of transferring glucose to an exogeneous acceptor, rt-dodecyl beta-D-maltoside. A small COOH-term inal region, conserved between G1g1p and G1g2p, is also important for function; mutation of Tyr(367) Or truncation at:residue 362 impairs th e ability of primed G1g2p to be elongated by glycogen synthase. Comple te suppression of glycogen accumulation in vivo requires mutation of a ll three Tyr residues. In G1g1p, two Tyr residues are implicated, Tyr( 232), and Tyr(600), mutation of both being required to eliminate glyco gen accumulation in vivo.