SACCHAROMYCES-CEREVISIAE PAC2 FUNCTIONS WITH CIN1, CIN2 AND CIN4 IN APATHWAY LEADING TO NORMAL MICROTUBULE STABILITY

The products of the Saccharomyces cerevisiae CIN1, CIN2 and CIN4 genes participate in a nonessential pathway required for normal microtubule function. In this article, we demonstrate that the product of PAC2 al so functions in this pathway. PAC2 deletion mutants displayed phenotyp es and genetic interactions similar to those caused by cin1 Delta, cin 2 Delta and cin4 Delta. These include cold-sensitive microtubule struc tures and sensitivity to the microtubule depolymerizing agent benomyl. Involvement in a common functional pathway is indicated by the observ ation that all double mutant combinations are viable and no more affec ted than any single mutant. In addition, extra copies of CIN1 were fou nd to suppress the benomyl sensitivity of pac2 Delta, cin2 Delta and c in4 Delta, but not that caused by other mutations that affect microtub ule function. Cin1p and Pac2p were found to be related in sequence to mammalian proteins that aid in the folding of beta-tubulin into an ass embly-competent state. Alleles of CIN1 were identified that could supp ress the benomyl sensitivity of cin4-4 in a highly specific fashion. O ur findings suggest that the guanine nucleotide-binding Cin4p interact s with Cin1p and regulates its tubulin folding activity.