PDS1P IS REQUIRED FOR FAITHFUL EXECUTION OF ANAPHASE IN THE YEAST, SACCHAROMYCES-CEREVISIAE

To identify mutations that cause defects in mitosis, a collection of m utants in Saccharomyces cerevisiae was screened by a rapid visual assa y for abnormal chromosome segregation, From this screen we identified one mutation, pds1-1 that was independently identified in an alternati ve screen for mutants that exhibit inviability after transient exposur e to nocodazole and precocious disassociation of sister chromatids (Gu acci, V., A. Yamamoto, A, Strunnikov, J. Kingsbury, E. Hogan, P. Meluh , and D. Koshland. 1993. CSH Symp, Quant. Biol. 58:677-685; Yamamoto, T.J., G. Li, B. Schaar, I. Szilak, and D.W. Cleveland. 1992. Nature (L ond.). 359:536-539). At 23 degrees C pds1-1 mutants exhibit frequent c ell death and a 300-fold increase in chromosome loss compared to wild type. At 37 degrees C pds1-1 cells fail to elongate their spindles dur ing anaphase. This spindle defect of pds1 mutants results from a tempe rature-sensitive step that occurs around the G1/S boundary about the t ime of spindle assembly. In the absence of spindle elongation pds1 mut ants undergo cytokinesis, leading to the missegregation of both chromo somes and spindle pole bodies. After abnormal cell division pds1-1 mut ants also initiate new rounds of DNA replication, spindle pole body du plication, and bud formation. Thus, in the pds1-1 mutant at 37 degrees C, cell cycle progression is uncoupled from the completion of anaphas e. A pds1 deletion allele has similar phenotypes to the original allel e. Taken together these results suggest that Pds1 protein plays an imp ortant role in chromosome segregation at 23 degrees C and an essential role for this process at 37 degrees C. The PDS1 gene encodes a novel 42-kD nuclear protein that has both basic and acidic domains. The leve l of PDS1 mRNA varies with the cell cycle with maximal accumulation ar ound the G1/S boundary. The stability of Pds1 protein also appears to change during the cell cycle as overproduced Pds1p is stable in S and M but degraded in early G1. Therefore, expression of Pds1p is regulate d apparently both transcriptionally and posttranslationally during the cell cycle. The phenotypes of pds1 mutants and expression pattern of Pds1p are discussed in the context of other spin die-defective mutants and the knowledge that Pds1 protein is an inhibitor of anaphase (Yama mote, T.J., G. Li, B. Schaar, I. Szilak, and D.W. Cleveland. 1992. Nat ure (Lond.). 359:536-539).