THE HIGH OSMOLARITY GLYCEROL RESPONSE (HOG) MAP KINASE PATHWAY CONTROLS LOCALIZATION OF A YEAST GOLGI GLYCOSYLTRANSFERASE

The yeast alpha-1,3-mannosyltransferase (Mnn1p) is localized to the Go lgi by independent transmembrane and lumenal domain signals. The lumen al domain is localized to the Golgi complex when expressed as a solubl e form (Mnn1-s) by exchange of its transmembrane domain for a cleavabl e signal sequence (Graham, T. R., and V. A. Krasnov. 1995. Mol. Biol. Cell. 6:809-824). Mutants that failed to retain the lumenal domain in the Golgi complex, called lumenal domain retention (ldr) mutants, were isolated by screening mutagenized yeast colonies for those that secre ted Mnn1-s. Two genes were identified by this screen, HOG1, a gene enc oding a mitogen-activated protein kinase (MAPK) that functions in the high osmolarity glycerol (HOG) pathway, and LDR1. We have found that b asal signaling through the HOG pathway is required to localize Mnn1-s to the Golgi in standard osmotic conditions. Mutations in HOG1 and LDR 1 also perturb localization of intact Mnn1p, resulting in its loss fro m early Golgi compartments and a concomitant increase of Mnn1p in late r Golgi compartments.