KAR1 has been shown to be required for both mitosis and conjugation (4). KAR1 was first isolated in a screen for mutations that block karyogamy (1). There are two basic steps in the process of karyogamy: nuclear congression and nuclear fusion (5). While the cell membranes of two cells fuse in response to mating pheromone, microtubules form to connect the two nuclei. The nuclei move toward one another; the spindle pole bodies (SPB) then fuse, followed by nuclear membrane fusion (5).

In kar1 mutant zygotes, nuclei remain separated by 3.3 microns, never becoming close enough to fuse (5). Temperature-sensitive kar1 mutants also exhibit a block in the initial stage of SPB duplication (4). Kar1p is associated with the half bridge of the SPB and is involved in localizing other proteins, including Cdc31p, Cik1p, and Kar3p, to the SPB (2, 6, 5). Separate protein domains of Kar1p are required for its role in karyogamy and SPB duplication (7, 8, 5). An internal 70 amino acid region is necessary and sufficient to localize a Kar1p-beta-galactosidase fusion protein to the SPB, and deletions in this region disrupt SPB duplication but do not cause defects in nuclear fusion. The amino terminal region is required only for nuclear fusion, while the carboxy terminal region functions in both SPB duplication as well as nuclear fusion and localizes the protein to the nuclear envelope (8). There is an excellent review by Marsh and Rose (5) describing cell and nuclear fusion during mating.

Last updated: 2000-04-07