A FRET Sensor to Monitor Bivalent SUMO-SIM Interactions in SUMO Chain Binding

The small ubiquitin-like modifier (SUMO) can be assembled into polymeric chains as part of its diverse biochemical signal pattern upon conjugation to substrate proteins. SUMO chain recognition is facilitated by receptor proteins that contain at least two SUMO-interacting motifs (SIMs). Little is known about the structure of SUMO chains, both in an unliganded form and upon complexation with multi-SIM protein partners. A FRET sensor has been developed based on a linear dimer of human SUMO-2 as a minimal SUMO chain analogue. The synthetic acceptor and donor dyes were conjugated by maleimide and copper-catalyzed click chemistry to each of the two SUMO subunits. FRET changes were only observed in the presence of di- or multi-SIM ligands. Alteration of the short linker sequence between SIMs 2 and 3 of RNF4 showed a great tolerance, and hence, structural flexibility, of the SUMO dimer for bivalent binding of adjacent SIMs. The di-SUMO FRET sensor reports on the binding of SIM clusters of the proteins C5orf25 and SOBP; this suggest that these can bind to adjacent subunits of a SUMO chain. The developed FRET sensor will be a useful tool to study the importance of SIM and linker sequences, as well as biochemical and structural properties of SUMO chains and multi-SIM proteins.