Abstract
Cyclin G2 (CCNG2) is an atypical cyclin that functions to inhibit cell cycle progression and is often dysregulated in human cancers. We have previously shown that cyclin G2 is highly unstable and can be degraded through the ubiquitin/proteasome pathway. Furthermore, cyclin G2 contains a PEST domain, which has been suggested to act as a signal for degradation by multiple proteases. In this study, we determined if calpains, a family of calcium-dependent proteases, are also involved in cyclin G2 degradation. The addition of calpain inhibitors or silencing of calpain expression by siRNAs strongly enhanced cyclin G2 levels. On the other hand, incubation of cell lysates with purified calpains or increasing the intracellular calcium concentration resulted in a decrease in cyclin G2 levels. Interestingly, the effect of calpain was found to be dependent on the phosphorylation of cyclin G2. Using a kinase inhibitor library, we found that Epidermal Growth Factor (EGF) Receptor is involved in cyclin G2 degradation and treatment with its ligand, EGF, induced cyclin G2 degradation. In addition, the presence of the PEST domain is necessary for calpain and EGF action. When the PEST domain was completely removed, calpain or EGF treatment failed to trigger degradation of cyclin G2. Taken together, these novel findings demonstrate that EGF-induced, calpain-mediated proteolysis contributes to the rapid destruction of cyclin G2 and that the PEST domain is critical for EGF/calpain actions.
Highlights
Cyclins encompass a group of closely related molecules
We showed that the PEST domain is critically involved in the stability of cyclin G2
Since multiple degradative mechanisms can act on a single protein, and several studies have reported that PEST sequence can be targeted by calpain for degradation [34, 39], we investigated if cyclin G2 can be degraded by the calpain
Summary
Cyclins encompass a group of closely related molecules. Classical cyclins accumulate periodically to activate their associated cyclin-dependant kinases (Cdk) and stimulate the mitotic events that regulate the rate of cell division [1]. The concentration of cyclins oscillate during the cell cycle [2], which allows for the strong unidirectional flow of cellular division. Cyclin expression levels are regulated by highly orchestrated protein turnover events and many cyclins are targeted for rapid degradation due to the presence of a destruction box or a PEST domain [3,4,5]. The Ubiquitin-Proteasome Pathway (UPP) is critical for the degradation of many short-lived proteins; and all known cyclins are targeted by the UPP [2, 3, 6, 7].
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