Effects of deletion mutations in the yeast Ces1 protein on cell growth and morphology and on high copy suppression of mutations in mRNA capping enzyme and translation initiation factor 4A

The homologous Saccharomyces cerevisiae genes CES1 and CES4 act as high copy suppressors of temperature-sensitive mutations of Ceg1p, the yeast mRNA capping enzyme. Neither CES1 nor CES4 is essential for cell growth. We find that a double deletion mutant (Delta ces1 Delta ces4) grows at 25-37 degrees C, but not at 16 degrees C. Delta ces1 Delta ces4 cells display gross defects in cell shape and budding even at permissive temperatures. Functional analysis of CES1 deletion mutants defines a 145 amino acid C-terminal segment of the 915 amino acid Ces1 protein that is necessary and sufficient to complement the Delta ces1 Delta ces4 cold-sensitive phenotype, to restore normal morphology and to suppress the temparature-sensitive mutant ceg1-25. A 147 amino acid C-terminal segment of the 942 amino acid Ces4 protein is sufficient to carry out these same functions. Within this carboxyl domain Ces1p and Ces4p are 80% identical to one another. We report isolation of CES1 in a separate screen for high copy suppression of a temperature-sensitive mutation (A79V) of the yeast translation initiation factor Tif1p (elF-4A). Deletion of the N-terminal 249 amino acids of Ces1p abolished tif1-A79V suppressor function. CES4 on a multicopy plasmid was unable to suppress tif1-A79V. We surmise that whereas the carboxyl domains of Ces1p and Ces4p are functionally redundant in controlling cell morphology and in suppressing ceg1-25, full-length Ces1p and Ces4p evince distinct genetic interactions that are likely mediated by their N-terminal segments.