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Group Leader
Group Leader since 2000 At the Max Planck Institute of Biochemistry since 2000 |
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Group Leader
Group Leader since 2000 At the Max Planck Institute of Biochemistry since 2000 |
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Post-translational
Modification with SUMO
INTRODUCTIONUbiquitin-like protein modification systems control a wide variety of cellular key processes. The most prominent member of the ubiquitin-like family is SUMO (Small Ubiquitin-like Modifier), whose functions include the regulation of transcriptional programs, genome stability and cell cycle progression (for review see Muller et al., 2004). Although the molecular details and functional consequences of SUMO modification on most target proteins are still not fully understood, modification by SUMO appears to alter protein functions by regulating specific protein/protein interactions and recent data indicate that signalling through SUMO occurs by non-covalent interactions between SUMO and specialized SUMO-interaction domains. Human cells express three SUMO forms (SUMO1, 2 and 3). SUMO2 and SUMO3 are highly similar and share an identity of ~97%, while SUMO1 and SUMO2/3 are ~43% identical. Conjugation of all SUMO forms to a target protein proceeds by a multi-step enzymatic pathway, which requires the E1 activating enzyme (Aos1/Uba2), the E2 conjugating enzyme Ubc9 and at least in some cases involves E3 ligases, such as protein inhibitor of activated STATs (PIAS) family members or Ran binding protein 2 (RanBP2). Importantly, SUMO modification is a highly dynamic and reversible process and the abundance of a given SUMO-conjugate is dynamically regulated by a balance between SUMO modification mediated by the E1, E2, E3 enzymes and demodification, which is governed by highly active SUMO proteases (SENPs). In mammalian cells six members of this faimly have been identified. However, their substrate specificity and specific cellular functions are largely unclear.
The pathway of SUMO conjugation/deconjugation. SUMO is synthesised as a precursor and C-terminally processed (arrowhead). Subsequently, the conjugation to proteins involves the E1 SUMO activating enzyme (AOS1/UBA2) and an E2 conjugating enzyme (Ubc9) that form thioesters (S) with the modifier. E3-like factors, like PIAS and RanBP2, stimulate the attachment to lysine residues within a target protein. The cleavage of SUMO from its target proteins, termed “de-sumoylation”, is catalysed by members of the SENP isopeptidase family. In humans six members of this family were identified.
FUTURE PROJECTS AND GOALS
The principal goal of our work is to decipher selected regulatory pathways in mammalian cells, which are controled by the SUMO system.
Currently we focus on the following topics
(1) Understanding the molecular features of SUMO-dependent protein/protein interactions – Impact on the control of cell signaling pathways
Recent data indicate that signaling through SUMO occurs by non-covalent interactions between SUMO and specialized SUMO-interaction motifs (SIM). SIMs are present in various proteins, including PML or PIAS family members. Using PIAS1 and PML as model proteins we could define an extended phosphoSIM module, which mediates binding to SUMO in a phosphorylation-dependent mode (Stehmeier and Müller, 2009). We could further identify Casein kinase 2 as the critical kinase involved in this process. This work represents a significant advancement in the molecular dissection of SUMO-regulated protein-protein interactions. Our current and future aim is to understand how this interconnection of the SUMO system to CK2 signaling regulates cellular pathways, in particular related to PML function and the PIAS/p53 axis.
(2) The SUMO System in the control of nucleolar dynamics and function
Our recent work has revealed a crucial function of the SUMO system in ribosome biogenesis and in particular shows an essential role of SENP3 in the conversion of the 32S rRNA intermediate to the mature 28S rRNA, with nucleophosmin (NPM1) being one critical SENP3 substrate in this process (Haindl et al., 2008). Since eukaryotic ribosome synthesis is a tightly controlled multi-step process that requires the coordinated action of a series of cellular components, we anticipate that proper processing of 32S rRNA involves SENP3-mediated desumoylation of additional factors in this pathway. In our current work we therefore address the following questions:
1. What are the cellular targets of SENP3 in ribosome biogenesis?
2. How does sumoylation affect the function of nucleolar proteins?
3. How is the tumour suppressor p14ARF connected to the nucleolar SUMO system?
(3) Mitotic functions of the SUMO system
Genetic studies in lower eukaryotes point to an involvement of the SUMO system in mitotic entry or progression. For example, in S. cerevisiae the conjugating enzyme Ubc9 is required for G2/M transition. Similarly, yeast mutants in the genes for the isopeptidase Ulp1 predominately arrest at the G2/M boundary of the cell division cycle indicating that a balanced equilibrium of conjugation and deconjugation is critical for entry into mitosis. However, in mammalian cells mitotic substrates of sumoylation and the regulatory components involved are not well defined. Our recent work revealed a critical function of the SUMO-specific isopeptidases SENP3 in mitosis.
Current and future work in this project will focus on the following key questions:
1. What are the mitotic targets of SENP3 and other isopeptidases?
2. How do sumoylation-desumoylation cycles affect protein function in mitosis?
3. How are SUMO isopeptidases regulated during mitosis?
