The essential N terminus of the pta1 scaffold protein is required for snoRNA transcription termination and Ssu72 function but is dispensable for pre-mRNA 3'-end processing

Saccharomyces cerevisiae Ptal is a component of the cleavage/ polyadenylation factor (CPF) 3'-end processing complex and functions in pre-mRNA cleavage, poly(A) addition, and transcription termination. In this study, we investigated the role of the N-terminal region of Ptal in transcription and processing. We report that a deletion of the first 75 amino acids (ptal-Δ75) causes thermosensitive growth, while the deletion of an additional 25 amino acids is lethal. Theptal-Δ75 mutant is defective for snoRNA termination, RNA polymerase II C-terminal domain Ser5-P dephosphorylation, and gene looping but is fully functional for mRNA 3'-end processing. Furthermore, different regions of Ptal interact with the CPF subunits Ssu72, Ptil, and Yshl, supporting the idea that Ptal acts as a scaffold to organize CPF. The first 300 amino acids of Ptal are sufficient for interactions with Ssu72, which is needed for pre-mRNA cleavage. By the degron-mediated depletion of Ptal, we show that the removal of this essential region leads to a loss of Ssu72, yet surprisingly, in vitro cleavage and polyadenylation remain efficient. In addition, a fragment containing amino acids 1 to 300 suppresses 3'-end processing in wild-type extracts. These findings suggest that the amino terminus of Ptal has an inhibitory effect and that this effect can be neutralized through the interaction with Ssu72.