The role of Saccharomyces cerevisiae Cdc40p in DNA replication and mitotic spindle formation and/or maintenance.

Successful progression through the cell cycle requires the coupling of mitotic spindle formation to DNA replication. In this report we present evidence suggesting that, in Saccharomyces cerevisiae, the CDC40 gene product is required to regulate both DNA replication and mitotic spindle formation. The deduced amino acid sequence of CDC40 (455 amino acids) contains four copies of a beta-transducin-like repeat. Cdc40p is essential only at elevated temperatures, as a complete deletion or a truncated protein (deletion of the C-terminal 217 amino acids in the cdc40-1 allele) results in normal vegetative growth at 23 degrees C, and cell cycle arrest at 36 degrees C. In the mitotic cell cycle Cdc40p is apparently required for at least two steps: (1) for entry into S phase (neither DNA synthesis, nor mitotic spindle formation occurs at 36 degrees C and (2) for completion of S-phase (cdc40::LEU2 cells cannot complete the cell cycle when returned to the permissive temperature in the presence of hydroxyurea). The role of Cdc40p as a regulatory protein linking DNA synthesis, spindle assembly/maintenance, and maturation promoting factor (MPF) activity is discussed.