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

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Abstract
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 (Δces1 Δces4) grows at 25–37°C, but not at 16°C. Δces1 Δces4 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 Δces1 Δces4 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 (eIF-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.

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