title: Viral oncoproteins E1A and E7 and cellular LxCxE proteins repress SUMO modification of the retinoblastoma tumor suppressor.
abstract: 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.
authors.affiliation: Department of Molecular Cell Biology, Max Planck Institute of Biochemistry, Am Klopferspitz 18, D-82152 Martinsried, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Ledl
authors.forename: Darja
authors.initials: D
authors.lastname: Schmidt
authors.forename: Stefan
authors.initials: S
authors.lastname: Müller
mesh_terms: D017500:Adenovirus E1A Proteins / Q000502:physiology*; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D018797:Cell Cycle Proteins / Q000235:genetics; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000235:genetics; D050684:E2F Transcription Factors; D006801:Humans; D008239:Lysine / Q000378:metabolism; D008969:Molecular Sequence Data; D009856:Oncogene Proteins, Viral / Q000502:physiology*; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D016160:Retinoblastoma Protein / Q000378:metabolism*; D025842:SUMO-1 Protein / Q000378:metabolism*; D014157:Transcription Factors / Q000235:genetics
chemical_list: D017500:Adenovirus E1A Proteins; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D009856:Oncogene Proteins, Viral; D016160:Retinoblastoma Protein; D025842:SUMO-1 Protein; D014157:Transcription Factors; D008239:Lysine