THE FUNCTIONING OF THE YEAST GOLGI-APPARATUS REQUIRES AN ER PROTEIN ENCODED BY ANP1, A MEMBER OF A NEW FAMILY OF GENES AFFECTING THE SECRETORY PATHWAY

Mnt1p is an alpha 1,2-mannosyltransferase which resides in an early co mpartment of the Saccharomyces cerevisiae Golgi apparatus. We have sho wn that the signal-anchor region is sufficient, and the transmembrane domain necessary, for its normal Golgi localization. This is similar t o the transmembrane domain-mediated retention of mammalian glycosyltra nsferases, and distinct from the tail-mediated recycling retention of certain mammalian and yeast trans-Golgi proteins. To examine the mecha nism involved in transmembrane domain-mediated retention, we have isol ated six classes of mutants which fail to retain Mnt1p-reporter fusion s in the early Golgi. These mutants all show additional phenotypes whi ch are consistent with alterations in Golgi function. We have called t he mutant classes 'gem', for Golgi enzyme maintenance. GEM3 is identic al to the previously cloned gene ANP1, and homologous to VAN1 and MNN9 . Together, these define a new class of proteins involved in the organ ization and functioning of the secretory pathway. Interestingly, Anp1p is localized to the endoplasmic reticulum (ER), implying that some fu nction of the ER is required to maintain a functional Golgi apparatus.