CAF1 plays an important role in mRNA deadenylation separate from its contact to CCR4

The CAF1 protein is a component of the CCR4 - NOT deadenylase complex. While yeast CAF1 displays deadenylase activity, this activity is not required for its deadenylation function in vivo, and CCR4 is the primary deadenylase in the complex. In order to identify CAF1- specific functional regions required for deadenylation in vivo, we targeted for mutagenesis six regions of CAF1 that are specifically conserved among CAF1 orthologs. Defects in residues 213 - 215, found to be a site required for binding CCR4, reduced the rate of deadenylation to a lesser extent and resulted in in vivo phenotypes that were less severe than did defects in other regions of CAF1 that displayed greater contact to CCR4. These results imply that CAF1, while affecting deadenylation through its contact to CCR4, has functions in deadenylation separate from its contact to CCR4. Synthetic lethalities of caf1 Delta, but not that of ccr4 Delta, with defects in DHH1 or PAB1, both of which are involved in translation, further supports a role of CAF1 separate from that of CCR4. Importantly, other mutations in PAB1 that reduced translation, while not affecting deadenylation by themselves or when combined with ccr4 Delta, severely blocked deadenylation when coupled with a caf1 deletion. These results indicate that both CAF1 and factors involved in translation are required for deadenylation.