SIC1 is a non-essential gene encoding a CDK inhibitor of Cdc28-Clb kinase activity, Sic1p is involved in both mitotic exit and the timing of DNA synthesis, To identify other genes involved in controlling Clb-kinase activity, we have undertaken a genetic screen for mutations which render SIC1 essential, Here we describe a gene we have identified by this means, RSI1/APC2. Temperature-sensitive rsi1/apc2 mutants arrest in metaphase and are unable to degrade Clb2p, suggesting that Rsi1p/Apc2p is associated with the anaphase promoting complex (APC), This is an E3 ubiquitin-ligase that controls anaphase initiation through degradation of Pds1p and mitotic exit via degradation of Clb cyclins, Indeed, the anaphase block in rsi1/apc2 temperature-sensitive mutants is overcome by removal of PDS1, consistent with Rsi1p/Apc2p being part of the APC, In addition, like our rsi1/apc2 mutations, cdc23-1, encoding a known APC subunit, is also lethal with sic1 Delta. Thus SIC1 clearly becomes essential when APC function is compromised, Finally, we find that Rsi1p/Apc2p co-immunoprecipitates with Cdc23p, Taken together, our results suggest that RSI1/APC2 is a subunit of APC.
