Interaction between Not1p, a component of the Ccr4-Not complex, a global regulator of transcription, and Dhh1p, a putative RNA helicase

The Ccr4-Not complex is a global regulator of transcription that affects genes positively and negatively and is thought to regulate transcription factor IID function. Two components of this complex, Caf1p and Ccr4p, are directly involved in mRNA deadenylation, and Caf1p is associated with Dhh1p, a putative RNA helicase thought to be a component of the decapping complex. In this work, we tried to determine whether Dhh1p, might interact with the Ccr4-Not complex. We found that, first, not mutations displayed severe synthetic phenotypes when combined with a dhh1-null mutation. Second, overexpression of Not1p was toxic in dhh1-null cells. Third, a not mutant phenotype was suppressed by deletion of DHH1 and mimicked by overexpression of DHH1. Fourth, dhh1-null mutants displayed resistance to heat shock, a phenotype observed for all mutants that affect the Ccr4-Not complex. Finally, like Caf1p and Ccr4p, Dhh1p co-immunoprecipitated with the nonessential N-terminal domain of Not1p, and the levels of Caf1p and Dhh1p were dependent upon this Not1p domain. Taken together, our results suggest that the Ccr4-Not complex, via the N-terminal region of Not1p, is necessary for the maintenance of stable cellular levels of Dhh1p and Caf1p, thus contributing to regulation of mRNA decay in addition to transcription.