SORTING OF YEAST ALPHA-1,3 MANNOSYLTRANSFERASE IS MEDIATED BY A LUMENAL DOMAIN INTERACTION, AND A TRANSMEMBRANE DOMAIN SIGNAL THAT CAN CONFER CLATHRIN-DEPENDENT GOLGI LOCALIZATION TO A SECRETED PROTEIN

alpha 1,3 mannosyltransferase (Mnn1p) is a type II integral membrane p rotein that is localized to the yeast Golgi complex. We have examined the signals within Mnn1p that mediate Golgi localization by expression of fusion proteins comprised of Mnn1p and the secreted protein invert ase. The N-terminal transmembrane domain (TMD) of Mnn1p is sufficient to localize invertase to the Golgi complex by a mechanism that is not saturable by similar to 15-20 fold overexpression. Furthermore, the TM D-mediated localization mechanism is clathrin dependent, as an inverta se fusion protein bearing only the Mnn1p TMD is mislocalized to the pl asma membrane of a clathrin heavy chain mutant. The Mnn1-invertase fus ion proteins are not retained in the Golgi complex as efficiently as M nn1p, suggesting that other signals may be present in the wild-type pr otein. Indeed, the Mnn1p lumenal domain (Mnn1-s) is also localized to the Golgi complex when expressed as a functional, soluble protein by e xchanging its TMD for a cleavable signal sequence. In contrast to the Mnn1-invertase fusion proteins, overexpression of Mnn1-s saturates its retention mechanism, and results in the partial secretion of this pro tein. These data indicate that Mnn1p has separable Golgi localization signals within both its transmembrane and lumenal domains.