Acetylation of SUMO paralogs as a determinant of SUMO-mediated protein networks

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The post-translational modification (PTM) of proteins through phosphorylation, acetylation or ubiquitylation is a central mechanism for the control of cellular signalling networks. A general principle is the selective recognition of the modified protein by a specific interaction domain. Modification by the ubiquitin-related SUMO protein also modulates specific protein-protein interactions thereby regulating various cellular signalling processes. Attachment of SUMO typically earmarks a protein for the recognition by a binding partner that contains a SUMO interaction motif (SIM). Prominent examples are the targeting of sumoylated RanGAP1 to RanBP2 or the SUMO/SIM-dependent assembly of PML nuclear bodies (NBs). Canonical SIMs are characterized by a stretch of hydrophobic residues, which interacts with a hydrophobic pocket on the surface of SUMO. In a subset of SIM modules acidic residues flank the hydrophobic core and enhance the binding to SUMO via electrostatic interactions with a basic interface on the modifier. Despite these relatively detailed molecular insights in SUMO/SIM interactions it is unclear how specificity and dynamics of these interactions are regulated. We recently unravelled the importance of PTMs for this process. We initially found that in a subset of SIM-containing proteins serine or threonine residues that juxtapose the hydrophobic region are phosphorylated. The additional negative charges of phosphorylated residues in these phosphoSIMs enhance the binding to the basic interface of SUMO. Very recently we defined site-specific acetylation of lysine residues within this basic interface as a novel regulatory principle in the control of SUMO/SIM interactions. We could show that acetylation of SUMO1 at K37 or SUMO2 at K33 still allows binding to RanBP2, but precludes the interaction with PIAS family members, PML or Daxx. Accordingly, SUMO affected the assembly of PML NBs and in particular prevented the SIM-dependent recruitment of the co-repressor Daxx to these structures. Importantly, in biochemical experiments we could further demonstrate that K33 acetylated SUMO2 serves as a binding platform for the Bromodomain (BRD) containing protein p300. In this proposal we aim to delineate how acetylation controls SUMO-mediated protein networks. We aim to identify specific substrates that are regulated by acetyl-SUMO modification and seek to get insight how this affects selected cellular pathways, in particular the control of gene expression. Finally, we want to unravel how the acetylation of SUMO is regulated.Within the funding period we anticipate to reach the following milestones:1. Decipher a specific acetyl-code on SUMO paralogs that determines the selectivity of SUMO/SIM and acetyl-SUMO/BRD interactions.2. Define targets of the acetyl-SUMO-code and delineate its impact on gene expression.3. Identify the regulatory components and physiological stimuli that control acetylation of SUMO paralogs.