Members of the RhoA subfamily of GTPases suppress the stimulatory effect of the HPV E6 oncoprotein on the ubiquitin ligase E6AP

The ubiquitin ligase E6AP, encoded by the UBE3A gene, plays a prominent role in human papillomavirus (HPV)-induced cancers. In complex with the HPV E6 oncoprotein, it targets inter alia the tumor suppressor p53 for degradation, thereby contributing to cellular transformation. Besides affecting the substrate spectrum of E6AP, E6 acts as a potent activator of the catalytic activity of E6AP. Here we report that RhoA and its subfamily members RhoB and RhoC serve as substrates of the E6-E6AP complex, but more strikingly, also act as strong inhibitors of the complex. We show that RhoA subfamily members potently inhibit E6-E6AP-mediated ubiquitination and degradation of p53 and that this inhibitory effect is not due to simple substrate competition. In addition to establishing RhoA subfamily members as the first known cellular regulators of the E6-E6AP complex, we provide evidence that RhoA binds to E6AP in the absence of E6. Notably, loss of functional E6AP is the cause of Angelman syndrome (AS), a neurodevelopmental disorder, and RhoA plays a critical role in maintaining synaptic structure and function. Thus, we propose that the interaction of RhoA subfamily members with E6AP is of significance for the development of both HPV-induced cancers and AS.