THE YEAST TEM1 GENE, WHICH ENCODES A GTP-BINDING PROTEIN, IS INVOLVEDIN TERMINATION OF M-PHASE

LTE1 belongs to the CDC25 family that encodes a guanine nucleotide exc hange factor for GTP-binding proteins of the ras family. Previously we have shown that LTE1 is essential for termination of M phase at low t emperatures. We have identified TEM1 as a gene that, when present on a multicopy plasmid, suppresses the cold-sensitive phenotype of Ite1. S equence analysis of TEM1 and GTP-binding analysis of the gene product revealed that TEM1 encodes a novel low-molecular-weight GTP-binding pr otein. The defect of TEM1 was lethal, and the tem1-defective cells wer e arrested at telophase with high H1-kinase activity under restrictive conditions, indicating that TEM1 is required to exit from M phase. Th e defect of TEM1 was suppressed by a high dose of CDC15, which encodes a protein kinase homologous to mitogen-activated protein kinase kinas e kinases. The genetic interaction among LTE1, TEM1, and CDC15 indicat es that they cooperatively play an essential role for termination of M phase.