Abstract

The SUMO proteins are Small Ubiquitin-related MOdifiers that are covalently conjugated onto lysine residues within target sequences. Invertebrates have a single SUMO gene, whereas the SUMO family in vertebrates have three members; SUMO-1, SUMO-2, and SUMO-3. The SUMO proteins are synthesised as inactive precursors, which are processed by SUMO-specific carboxy-terminal hydrolases, resulting in novel double-glycine C-termini. The mature SUMO proteins are then activated by Aos1/Uba2 an activating enzyme (E1), and transferred to Ubc9 a conjugating enzyme (E2). Eventually, the SUMO protein is covalently linked to the target protein by the formation of an isopeptide bond between the carboxyl terminus of SUMO and an epsilon-amino group of a lysine residue of the target protein. The reaction is aided by an E3 ligase, e.g. mammalian PIAS1. This process, termed sumoylation, is reversible.



Most sumoylated proteins are nuclear, and three main functional roles of SUMO have been proposed. (i) Protein targeting sumoylation has been shown to be important for nuclear import of the RanGAP1 protein, and for recruiting proteins to subnuclear protein complexes (e.g. promyelocytic leukemia protein (PML) to PML nuclear bodies). (ii) Enhancement of protein stability by potential competition with (and inhibition of) ubiquitination. (iii) Transcriptional control (e.g. negative regulation of transcription from the androgen receptor).



A core motif (PhiKxE) has been identified as the sumoylation target for SUMO-1 (11553772, 11121022, 10212234). SUMO-2/3 themselves contain a PhiKxE site, in contrast to SUMO-1, and can thus form polymeric chains (shown in vitro, and in vivo for SUMO-2) (11451954).



A number of reports in the literature also suggest non-canonical sumoylation sites. While many of these may be incorrect, some seem to be widely accepted. High resolution structural data would be the best way to verify genuine examples.



The SUMO site in C/EBP transcription factors shows an extended sequence conservation. A bioinformatics survey of nuclear proteins revealed a common extended SUMO site, termed the KEPE motif (19033273) in transcriptional and chromatin proteins. The function of the KEPE motif remains to be determined.



For SUMO reviews see e.g. 18492068, 18031233, 18000527