In yeast, which protein is the functional equivalent of Rb in regulating the G1/S transition?
In mammalian cells, Rb represses E2F until cyclin D-CDK4/6 phosphorylates pocket domain, but budding yeast Saccharomyces cerevisiae lacks Rb ortholog and uses functionally analogous repressor Whi5 to regulate Start checkpoint analogous to Restriction Point. Whi5 contains CDK phosphorylation sites and binds heterodimeric transcription factor SBF composed of Swi4 DNA-binding protein recognizing SCB elements and Swi6 coactivator. In early G1 when CDK activity low, unphosphorylated Whi5 resides in nucleus, physically interacting with SBF at promoters of CLN1, CLN2, CLB5, and many cell wall synthes
Ref: Cross et al., Annu Rev Cell Dev Biol 2007, Yeast G1/S Control. NCBI Bookshelf: Whi5 Rb Analog.