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 synthesis genes, recruiting histone deacetylases Hos3 and Rpd3 to block transcription. As cell grows, Cln3 cyclin synthesized proportionally to size binds Cdc28 CDK1 and phosphorylates Whi5, triggering export via Msn5 karyopherin. SBF liberated induces Cln1 and Cln2 that further phosphorylate Whi5 in positive feedback loop analogous to cyclin E-CDK2 hyperphosphorylating Rb. Nuclear exit defines commitment to replication and budding, illustrating convergent regulatory logic. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.
Ref: Cross et al., Annu Rev Cell Dev Biol 2007, Yeast G1/S Control. NCBI Bookshelf: Whi5 Rb Analog.