Functional role of SUMO modification in the retinoflastoma tumour suppressor pathway
Final Report Abstract
Cellular signaling pathways require the plasticity of multiprotein complexes. One important mechanism that ensures the coordinated assembly and disassembly of protein complexes is the reversible post-translational modification of their individual components for example by phosphorylation, acetylation or ubiquitylation. The ubiquitin-related SUMO system is another important organizer of protein complexes. Modification by SUMO (SUMOylation) targets a large number of cellular proteins and generally mediates protein/protein interactions through binding of SUMO-conjugates to a defined interaction module, termed SUMO interaction motif (SIM). Our project was based on the finding that the retinoblastoma tumor suppressor (pRb), which is an important regulator of cell-cycle progression and cell differentiation is modified by SUMO. The major aim was to unravel the functional significance of pRB SUMOylation and in particular to understand how SUMOylation controls binding to specific binding partner, including promyelocytic leukemia protein (PML). Within the funding period we faced some difficulties, which led us to re-adjust the focus of the project. One particular problem was that we could not detect SUMOylation of pRB at the endogenous expression level. Moreover, in several cell biological assays we did not observe phenotypic differences between cells expressing wild-type pRB or the SUMO-deficient variant K720R. Therefore we decided to more generally investigate the mechanism of SUMO/SIM-dependent protein/protein interactions. Using PML as one model protein we could define a novel sub-type of SIM, which requires CK2- mediated phosphorylation for potent binding to SUMO. Similar phospho-dependent SUMO binding modules, which we collectively termed phospho-SIMs, were also delineated in the SUMO E3 Ligase PIAS and the exosome component PMSCL-1. This suggests that phosphorylation represents an additional regulatory layer that controls SUMO binding to a specific subclass of SIMs. In another approach to understand the dynamics of SUMO-regulated complex formation we decided to characterize SUMO specific isopeptidases. We focussed our attention on the nucleloar isopeptidase SENP3 and could demonstrate that SENP3 is important for the proper formation of ribosomes. In particular we found that SENP3 is required for the de SUMOylation of nucleolar proteins that control the maturation of the 28S rRNA. Taken together, our results provide new insights in the regulation and function of the SUMO system. Although we did not succeed in elucidating the functional significance of pRB SUMOylation in this project, we were able to make novel contributions to the field.
Publications
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(2008) The Adenovirus E1B-55K Oncoprotein Induces SUMO Modification of p53, Cell Cycle, 7, 754-758
Müller S. & Dobner T.
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(2008) The nucleolar SUMO-specific protease SENP3 reverses SUMO modification of nucleophosmin and is required for rRNA processing, EMBO reports, 9, 273-279
Haindl M., Harasim T., Eick D. & Müller S.
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(2009) E6AP promotes the degradation of the PML tumour suppressor, Cell Death Diff., 16, 1156-1166
Louria-Hayon I., Alsheich-Bartok O., Levav-Cohen Y., Silberman I., Berger M. , Tamar Grossman T., Matentzoglu K., Jiang Y-H., Müller S., Scheffner M. , Haupt S. & Haupt Y.
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(2009) Phospho-regulated SUMO interaction modules connect the SUMO system to CK2 signaling, Molecular Cell, 30, 400-409
Stehmeier P. & Müller S.
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(2009) RanBP2 and SENP3 function in a mitotic SUMO2/3 conjugation-deconjugation cycle on Boralin, Mol. Biol. Cell, 20, 410-418
Klein U., Haindl M., Nigg E.A. & Müller S.
