INTERACTION WITH CALMODULIN IS REQUIRED FOR THE FUNCTION OF SPC110P, AN ESSENTIAL COMPONENT OF THE YEAST SPINDLE POLE BODY

NUF1/SPC110, encoding a nuclear filament-related protein which is a co mponent of the yeast spindle pole body (SPB), has been identified in a screen designed to isolate genes encoding targets of yeast calmodulin . Spc110p interacts with calmodulin by two different criteria and the calmodulin interacting region has been localized within the C-terminus of the protein. Point mutations between residues 898 and 917 further define the calmodulin binding site within this region. Mutations in th is domain which abolish calmodulin binding lit vitro prevent Spc110p f unction in vivo, demonstrating that calmodulin binding by Spc110p has important functional consequences. In keeping with a role for calmodul in in Spc110p function, we show that calmodulin localizes to the yeast SPB when cells are prepared under appropriate conditions. Nonfunction al mutant Spc110 proteins which cannot bind calmodulin are present at lowered steady-state levels in the cell; when their level is increased by elevated gene dosage, partial recovery of Spc110p function is seen . Overexpression of calmodulin suppresses the defect(s) associated wit h the mutant Spc110 proteins, supporting the notion that Spc110p stabi lity is a consequence of its ability to bind calmodulin and pointing t o a direct role for calmodulin in Spc110p function.