Mutations in SPC110, encoding the yeast spindle pole body calmodulin-binding protein, cause defects in cell integrity as well as spindle formation

The 110 kDa spindle pole body component, Spc110p, is an essential target of calmodulin in budding yeast. Cells with mutations which reduce calmodulin binding to Spc110p are unable to form a mitotic spindle and die. Here we show that these effects can be overcome either directly by increasing extracellular calcium or calmodulin expression, which reverse the primary spindle defect, or indirectly through increased extracellular osmolarity or high dosage of MID2 or SLG1/HCS77/WSC1 which preserve viability. We propose that overcoming a cell integrity defect associated with the mitotic arrest enables the defective spindle pole bodies to provide sufficient function for proliferation of a large proportion of mutant cells. Our findings demonstrate a role for calcium in the Spc110p-calmodulin interaction in vivo and have important general implications for the interpretation of genetic interactions involving cell integrity genes.