Viral oncoproteins E1A and E7 and cellular LxCxE proteins repress SUMO modification of the retinoblastoma tumor suppressor.

The retinoblastoma tumor suppressor protein (pRB) is a major regulator of cell-cycle progression and cellular differentiation. Central to pRB function is the pocket domain, which serves as the main binding region for cellular regulators. In tumors pRB is frequently inactivated by mutations in the pocket domain or by binding of viral oncoproteins to this region. A characteristic feature of these viral oncoproteins and many cellular pRB-binding partners is an LxCxE sequence motif, which interacts with pRB's pocket domain. Here, we show that the ubiquitin-like modifier SUMO is covalently attached to a distinct residue (K720) of pRB within the B-box of the pocket region that binds LxCxE-motif proteins. We provide evidence that SUMO preferentially targets the active, hypophosphorylated form of pRB and show that tumorigenic mutations of pRB in the pocket domain lead to a loss of SUMOylation. Notably, the level of pRB SUMOylation is controlled by the interaction of pRB with viral and cellular LxCxE-motif proteins. Inhibitors of pRB function, including the viral oncoproteins E1A and E7 and the cellular E1A-like inhibitor of differentiation EID-1, completely abolish SUMO modification of pRB. Conversely, pRB mutants deficient in binding of LxCxE-motif proteins exhibit a drastically enhanced modification by SUMO. Finally, we provide evidence that SUMOylation can influence pRB function, as the SUMO-deficient pRB(K720R) mutant exerts a slightly higher repressive potential on an E2F-responsive reporter gene than wild-type pRB. Taken together, these data identify SUMO modification as a novel post-translational modification of pRB that may control pRB activity by modulating LxCxE-pocket interactions.

SUMO: a regulator of gene expression and genome integrity.

Post-translational modification with the ubiquitin-like SUMO protein is involved in the regulation of many cellular key processes. The SUMO system modulates signal transduction pathways, including cytokine, Wnt, growth factor and steroid hormone signalling. SUMO frequently restrains the activity of downstream transcription factors in these pathways presumably by facilitating the recruitment of corepressors or mediating the assembly of repressor complexes. Additionally, evidence is accumulating that SUMO controls pathways important for the surveillance of genome integrity. SUMO regulates the PML/p53 tumour suppressor network, a key determinant in the cellular response to DNA damage. Moreover, proteins that maintain genomic stability by functioning at the interface between DNA replication, recombination and repair processes undergo SUMOylation. We will discuss some key findings that exemplify the role of SUMO in transcriptional regulation and genome surveillance.