Spatial regulation of the Start repressor Whi5

Abstract

The Saccharomyces cerevisiae Start repressor Whi5, the functional analogue of mammalian pRB, shuttles between the nucleus and the cytoplasm throughout the cell cycle: Enters into the nucleus at the end of mitosis and remains nuclear until Start. We studied the mechanisms involved in this spatial regulation. The nuclear import depends on the β-karyopherins of the classical import pathway Kap95 and Cse1. Whi5 contains a monopartite and a bipartite classical NLS localized in its N-terminal region which are functionally redundant. A fragment of Whi5 containing these NLSs is able to constitutively accumulate a GFP4 protein inside the nucleus throughout the cell cycle, which suggests that the Whi5 nuclear import is not cell cycle-regulated. The nuclear export of Whi5 is assisted by β-karyopherin Msn5. A two-hybrid assay indicates a physical interaction between Whi5 and Msn5. We identified a fragment of Whi5 with export activity from amino acids 51 to 167. Interestingly, this region drives the export of a chimeric nuclear protein in a cell cycle-regulated pattern similarly to that observed for Whi5. Moreover, the nuclear export driven by Whi5 depends on the phosphorylation of specific Ser residues. Finally, we identified Cdc14 as the phosphatase required for the nuclear accumulation of Whi5.