Involved in ubiquitination and subsequent proteasomal degradation of target proteins. Required for both entry into S phase and kinetochore function. Also involved in cyclic AMP (cAMP) pathway; possibly by participating in the assembly or the conformational activation of specific multiprotein complexes. {ECO:0000269|PubMed:10445024; ECO:0000269|PubMed:12456005}.