SUMO junction-what's your function? New insights through SUMO-interacting motifs.

The small ubiquitin-like modifier, SUMO, can be covalently linked to specific proteins and many substrates carrying this modification have been identified. However, for some proteins, the role that SUMO modification imparts remains obscure. Our understanding of SUMO biology and function has been significantly advanced by the recent discovery of proteins and protein domains that contain SUMO-interacting motifs (SIMs), which interact non-covalently with SUMO. Unlike the motifs and domains that mediate ubiquitin binding, the diversity of SIMs seems limited. Nevertheless, SIMs have already increased our understanding of how SUMO affects DNA repair, transcriptional activation, nuclear body formation and protein turnover. This review takes a detailed look at how SIMs were identified, how they specifically bind to SUMO, their crucial roles in multi-step enzymatic processes, and how they direct the assembly and disassembly of dimeric and multimeric protein complexes.