KAR1

Kar1 is a gene in the budding yeast Saccharomyces cerevisiae that encodes a spindle pole body–associated protein. The Kar1 protein localizes to the spindle pole body (SPB), the yeast equivalent of a centrosome, and functions mainly at the SPB half-bridge that links the two SPBs during duplication. Its activity is important for SPB duplication, proper formation of the mitotic spindle, and efficient nuclear congression during mating (karyogamy).

Genetic analyses have shown that the kar1-1 allele is temperature sensitive: at restrictive temperatures it blocks SPB duplication, producing cells with a single SPB and leading to mitotic arrest. Kar1 mutations also impair karyogamy, indicating a role in nuclear fusion during mating. Kar1p is thought to contribute to microtubule nucleation and anchoring by functioning with other SPB components, including centrin Cdc31 and additional half-bridge proteins.

The KAR1 gene is considered essential for viability because SPB duplication is indispensable for cell division. In addition to its essential background in S. cerevisiae, KAR1 is conserved among related fungi, reflecting a broader role in SPB structure and function across species.

Discovery and terminology: KAR1 stands for karyogamy 1 and was identified through genetic screens for defects in nuclear fusion during mating. In research, KAR1 continues to serve as a model for studying SPB assembly, spindle dynamics, and meiosis in yeast. The precise molecular interactions and regulatory mechanisms involving Kar1p remain under investigation, with ongoing work to map its partners and its coordination of SPB duplication with the cell cycle.