NUCLEAR CONGRESSION AND MEMBRANE-FUSION - 2 DISTINCT EVENTS IN THE YEAST KARYOGAMY PATHWAY

Karyogamy is the process where haploid nuclei fuse to form a diploid n ucleus during yeast mating. We devised a novel genetic screen that ide ntified five new karyogamy (KAR) genes and three new cell fusion (FUS) genes. The kar mutants fell into two classes that represent distinct events in the yeast karyogamy pathway. Class I mutations blocked congr ession of the nuclei due to cytoplasmic microtubule defects. In Class II mutants, nuclear congression proceeded and the membranes of apposed nuclei were closely aligned but unfused. In vitro, Class II mutant me mbranes were defective in a homotypic ER/nuclear membrane fusion assay . We propose that Class II mutants define components of a novel membra ne fusion complex which functions during vegetative growth and is recr uited for karyogamy.