Target of rapamycin proteins and their kinase activities are required for meiosis

The phosphatidylinositol kinase-related kinases, including Tor1p, Tor2p, FRAP/RAFT, FRP/ATR, ATM, Mec1p, Rad3, and Tel1p, function in signal transduction pathways involved in cell cycle progression and surveillance. The rapamycin-sensitive kinase activities of Tor1p and Tor2p are required for the nutrient-activated protein translation essential for G(1) cell cycle progression in haploid yeast cells. In addition, Tor2p’s kinase activity is necessary for its unique rapamycin-insensitive function involved in the assembly of the actin cytoskeleton. In the current study using diploid yeast, we found that the kinase activities of the Tor proteins are also required for two discrete steps during yeast meiosis—the switch between the mitotic and meiotic cell cycles and a later step during meiosis involved in the packaging of resultant haploid cells (spores) into asci. Based on what is known of the mitotic functions of Tor and FRAP proteins, these results likely reflect the requirement for signaling pathways leading to regulated protein translation during meiosis. Mec1p, which is required for meiotic recombination, and the Tor proteins are, therefore, homologous kinases with distinct, yet essential, roles in meiosis.