Characterization and functional analysis of the cyclin G1 protein

Cyclin G1 was identified as a gene with homology to the cyclin family of cell cycle regulators. Cyclins regulate cell cycle transitions via activation of cyclin-dependent kinases (cdks). Cyclin G1 is the only putative cyclin whose transcription is regulated by the p53 tumor suppressor protein. Although cyclin G1 was identified nearly 10 years ago, virtually nothing is known about its physical properties and knowledge of its biological function is scant and controversial.; My data indicate that the cyclin G1 protein is modified by ubiquitin and is unstable. The degradation of cyclin G1 is inhibited by proteasome inhibitors, by coexpression of inactive cdks and by ARF, a negative regulator of the p53-ubiquitinating enzyme MDM2. Conversely, ARF levels are reduced by overexpression of cyclin G1, leading to a decrease in ARF-mediated G1 arrest. Further, cyclin G1 binds MDM2 and overexpression of MDM2 enhances the ubiquitination of cyclin G1. Cyclin G1 is predominantly nucleoplasmic, but its localization is regulated by cdks, a cyclin box mutation and coexpression of ARF. My data support the notion that cyclin G1 has a role in the p53/MDM2/ARF pathway.; Cyclin G1 has been categorized as a cyclin because it contains a cyclin box motif, required for activation of cyclin-dependent kinase partners. Coexpression of cyclin G1 and several known cdks in mammalian and insect cells results in cyclin G1/cdk complex formation, but no detectable kinase activity. However, an endogenous cyclin G1-associated kinase activity was identified in insect cell lysates. Further, cyclin G1 itself is phosphorylated in kinase assays performed from insect cell lysates and in phosphate-labeled U2OS cells. These data suggest that cyclin G1 may activate an unidentified cdk resulting in an activity that regulates the stability of the cyclin G1 protein.; U2OS cells overexpressing cyclin G1 showed no striking alteration in cell cycle profile in the absence of DNA damage. However, an effect was seen after γ-irradiation of synchronized cells or exposure to a replication-defective adenovirus expressing cyclin G1. These data support a role for cyclin G1 in the DNA damage response and suggest that it may synergize with viral proteins to disregulate cell proliferation.