CLN3 expression is sufficient to restore G1-to-S-phase progression in Saccharomyces cerevisiae mutants defective in translation initiation factor eIF4E

The essential cap-binding protein (eIF4E) of Saccharomyces cerevisiae is encoded by the CDC33 (wild-type) gene, originally isolated as a mutant, cdc33-1, which arrests growth in the G1 phase of the cell cycle at 37 degrees C. We show that other cdc33 mutants also arrest in G1. One of the first events required for G1-to-S-phase progression is the increased expression of cyclin 3. Constructs carrying the 5'-untranslated region of CLN3 fused to lacZ exhibit weak reporter activity, which is significantly decreased in a cdc33-1 mutant, implying that CLN3 mRNA is an inefficiently translated mRNA that is sensitive to perturbations in the translation machinery. A cdc33-1 strain expressing either stable Cln3p (Cln3-1p) or a hybrid UBI4 5'-CLN3 mRNA, whose translation displays decreased dependence on eIF4E, arrested randomly in the cell cycle. In these cells CLN2 mRNA levels remained high, indicating that Cln3p activity is maintained. Induction of a hybrid UBI4 5'-CLN3 message in a cdc33-1 mutant previously arrested in G1 also caused entry into a new cell cycle. We conclude that eIF4E activity in the G1-phase is critical in allowing sufficient Cln3p activity to enable yeast cells to enter a new cell cycle.

Isolation of a protein complex containing translation initiation factor Prt1 from Saccharomyces cerevisiae

Translation initiation factor Prt1 was purified from a ribosomal salt wash fraction of Saccharomyces cerevisiae cells by ammonium sulfate precipitation, DEAE chromatography, phosphocellulose chromatography, sucrose density gradient centrifugation, and non-denaturing polyacrylamide gel electrophoresis. Prt1 protein cofractionates with four other polypeptides during all steps of purification suggesting that it is part of a protein complex containing polypeptide subunits with apparent molecular masses of 130, 80, 75 (Prt1), 40, and 32 kDa. Deletion of the first AUG codon in the published sequence of the PRT1 gene results in the synthesis of functional Prt1 protein indicating that the actual molecular mass of the Prt1 subunit is 82.7 kDa. This is in agreement with results from primer extension experiments reported earlier by Keierleber et al. (Keierleber, C., Wittekind, M., Qin, S., and McLaughlin, C. S. (1986) Mol. Cell. Biol. 6, 4419-4424). The Prt1-containing protein complex is an active translation factor as shown by its ability to restore translation in a cell-free system derived from a temperature-sensitive prt1 mutant strain in which endogenous Prt1 activity is inactivated by heating the extract to 37 degrees C. The question of whether the Prt1-containing protein complex represents the yeast homologue of mammalian translation initiation factor eIF-3 is discussed.