Gip2p is a nonessential protein involved in glycogen accumulation, and may be a regulatory subunit of the Glc7p type-1 protein phosphatase (PP1) (4, 2). Gip2p displays similarity to Gac1p, which is a regulatory subunit for Glc7p, and also to Pig1p and Pig2p, which bind the Gsy2p glycogen synthase in two-hybrid assays (4, 1, 2). Indeed, Gip2p, Gac1p, Pig1p, and Pig2p are the only four proteins in the yeast genome that share a conserved segment of approximately 25 residues, designated the GVNK motif, that is identifiable also in RGI, the mammalian type 1 phosphatase targeting subunit (4). Two-hybrid analyses indicate that Gip2p does in fact bind Glc7p, as well as a truncated species of the Ppz1p protein phosphatase (Ppz1p-[361-692]) (1, 5).

GIP2 is induced in response to ethanol shock (6). GIP2 induction has also been demonstrated in three of three optimally adapted strains from independent cultures after continuous aerobic growth for more than 250 generations in glucose-limited chemostats, suggesting that increased expression of GIP2 is associated with increased fitness during conditions of limited glucose (7). Although Gip2 null mutants are viable (4), they do accumulate decreased levels of glycogen. Also, a gip2 null glc7-109 double mutant accumulates less glycogen than a glc7-109 mutant alone (1).

Gip2p possesses two short regions of similarity to the Rhizopus glucoamylase precursor AMYG, and also to various mammalian glycogen-associated regulatory subunits of protein phosphatase 1 (4), including rabbit RG RAB and human PPP1R3, mutations in which are associated with non-insulin-dependent diabetes mellitus (1).

Last updated: 2005-10-13