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LTE1 belongs to the CDC25 family that encodes a guanine nucleotide exchange factor for GTP-binding proteins of the ras family. Previously we have shown that LTE1 is essential for termination of M phase at low temperatures. We have identified TEM1 as a gene that, when present on a multicopy plasmid, suppresses the cold-sensitive phenotype of lte1. Sequence analysis of TEM1 and GTP-binding analysis of the gene product revealed that TEM1 encodes a novel low-molecular-weight GTP-binding protein. The defect of TEM1 was lethal, and the tem1-defective cells were arrested at telophase with high H1-kinase activity under restrictive conditions, indicating that TEM1 is required to exit from M phase. The defect of TEM1 was suppressed by a high dose of CDC15, which encodes a protein kinase homologous to mitogen-activated protein kinase kinase kinases. The genetic interaction among LTE1, TEM1, and CDC15 indicates that they cooperatively play an essential role for termination of M phase.