THE SAPS, A NEW FAMILY OF PROTEINS, ASSOCIATE AND FUNCTION POSITIVELYWITH THE SIT4 PHOSPHATASE

SIT4 is the catalytic subunit of a type 2A-related protein phosphatase in Saccharomyces cerevisiae that is required for G(1) cyclin transcri ption and for bud formation. SIT4 associates with several high-molecul ar-mass proteins in a cell cycle-dependent fashion. We purified two SI T4-associated proteins, SAP155 and SAP190, and cloned the correspondin g genes. By sequence homology, we isolated two additional SAP genes, S AP185 and SAP4. Though such an association is not yet proven for SAP4, each of SAP155, SAP185, and SAP190 physically associates with SIT4 in separate complexes. The SAPs function positively with SIT4, and by se veral criteria, the loss of all four SAPs is equivalent to the loss of SIT4. The data suggest that the SAPs are not functional in the absenc e of SIT4 and likewise that SIT4 is not functional in the absence of t he SAPs. The SAPs are hyperphosphorylated in cells lacking SIT4, raisi ng the possibility that the SAPs are substrates of SIT4. By sequence s imilarity, the SAPs fall into two groups, the SAP4/SAP155 group and th e SAP185/SAP190 group. Overexpression of a SAP from one group does not suppress the defects due to the loss of the other group. These findin gs and others indicate that the SAPs have distinct functions.