Nuclear congression is driven by cytoplasmic microtubule plus end interactions in S. cerevisiae

Nuclear movement before karyogamy in eukaryotes is known as pronuclear migration or as nuclear congression in Saccharomyces cerevisiae. In this study, S. cerevisiae is used as a model system to study microtubule (MT)-dependent nuclear movements during mating. We find that nuclear congression occurs through the interaction of MT plus ends rather than sliding and extensive MT overlap. Furthermore, the orientation and attachment of MTs to the shmoo tip before cell wall breakdown is not required for nuclear congression. The MT plus end-binding proteins Kar3p, a class 14 COOH-terminal kinesin, and Bik1p, the CLIP-170 orthologue, localize to plus ends in the shmoo tip and initiate MT interactions and depolymerization after cell wall breakdown. These data support a model in which nuclear congression in budding yeast occurs by plus end MT capture and depolymerization, generating forces sufficient to move nuclei through the cytoplasm. This is the first evidence that MT plus end interactions from oppositely oriented organizing centers can provide the force for organelle transport in vivo.