GENETIC INTERACTIONS BETWEEN CDC31 AND KAR1, 2 GENES REQUIRED FOR DUPLICATION OF THE MICROTUBULE-ORGANIZING CENTER IN SACCHAROMYCES-CEREVISIAE

KAR1 encodes an essential component of the yeast spindle pole body (SP B) that is required for karyogamy and SPB duplication. A temperature-s ensitive mutation, kar1-Delta 17 mapped to a region required for SPB d uplication and for localization to the SPB. To identify interacting SP B proteins, we isolated 13 dominant mutations and 3 high copy number p lasmids that suppressed the temperature sensitivity of kar1-Delta 17. Eleven extragenic suppressor mutations mapped to two linkage groups, D SK1 and DSK2. The extragenic suppressors were specific for SPB duplica tion and did not suppress kargogamy-defective alleles. The major class , DSK1, consisted of mutations in CDC31. CDC31 is required for SPB dup lication and encodes a calmodulin-like protein that is most closely re lated to caltractin/centrin, a protein associated with the Chlamydomon as basal body. The high copy number suppressor plasmids contained the wild-type CDC31 gene. One CDC31 suppressor allele conferred a temperat ure-sensitive defect in SPB duplication, which was counter-suppressed by recessive mutations in KAR1. In spite of the evidence for a direct interaction, the strongest CDC31 alleles, as well as both DSK2 alleles , suppressed a complete deletion of KAR1. However, the CDC31 alleles a lso made the cell supersensitive to KAR1 gene dosage, arguing against a simple bypass mechanism of suppression. We propose a model in which Kar1p helps localize Cdc31p to the SPB and that Cdc31p then initiates SPB duplication via interaction with a downstream effector.