Microtubule dynamics in the budding yeast mating pathway.

In order for haploid gametes to fuse during fertilization, microtubules (MTs) must generate forces that are sufficient to move the nuclei together. Nuclear movements during fertilization rely on microtubule-associated proteins (MAPs), many of which have been characterized extensively during mitosis. A useful model system to study MT-dependent forces before nuclear fusion, or karyogamy, is the mating pathway of budding yeast. Dynamic MTs are guided to the mating projection (shmoo tip) when plus-end-binding proteins interact with polarized actin microfilaments. If two shmoo tips are in proximity they may fuse, dissolving the MT-cortical interactions. Subsequently, oppositely oriented MT plus ends interact and draw the nuclei together. The plus-end-binding proteins in the yeast mating pathway are conserved in metazoan cells and may play a role in higher eukaryotic fertilizaton. Thus, understanding the mechanism of plus end orientation and karyogamy in budding yeast will reveal mechanisms of MT-dependent force generation conserved throughout evolution.