The unusual UBZ domain of Saccharomyces cerevisiae polymerase η.

Recent research has revealed the presence of ubiquitin-binding domains in the Y family polymerases. The ubiquitin-binding zinc finger (UBZ) domain of human polymerase η is vital for its regulation, localization, and function. Here, we elucidate structural and functional features of the non-canonical UBZ motif of Saccharomyces cerevisiae pol η. Characterization of pol η mutants confirms the importance of the UBZ motif and implies that its function is independent of zinc binding. Intriguingly, we demonstrate that zinc does bind to and affect the structure of the purified UBZ domain, but is not required for its ubiquitin-binding activity. Our finding that this unusual zinc finger is able to interact with ubiquitin even in its apo form adds support to the model that ubiquitin binding is the primary and functionally important activity of the UBZ domain in S. cerevisiae polymerase η. Putative ubiquitin-binding domains, primarily UBZs, are identified in the majority of known pol η homologs. We discuss the implications of our observations for zinc finger structure and pol η regulation.