The Small Ubiquitin-like Modifier (SUMO) Protein Modification System in Arabidopsis

Abstract

Small ubiquitin-like modifier (SUMO) is a member of the superfamily of ubiquitin-like polypeptides that become covalently attached to various intracellular target proteins as a way to alter their function, location, and/or half-life. Here we show that the SUMO conjugation system operates in plants through a characterization of the Arabidopsis SUMO pathway. An eight-gene family encoding the SUMO tag was discovered as were genes encoding the various enzymes required for SUMO processing, ligation, and release. A diverse array of conjugates could be detected, some of which appear to be SUMO isoform-specific. The levels of SUMO1 and -2 conjugates but not SUMO3 conjugates increased substantially following exposure of seedlings to stress conditions, including heat shock, H 2 O 2 , ethanol, and the amino acid analog canavanine. The heat-induced accumulation could be detected within 2 min from the start of a temperature upshift, suggesting that SUMO1/2 conjugation is one of the early plant responses to heat stress. Overexpression of SUMO2 enhanced both the steady state levels of SUMO2 conjugates under normal growth conditions and the subsequent heat shock-induced accumulation. This accumulation was dampened in an Arabidopsis line engineered for increased thermotolerance by overexpressing the cytosolic isoform of the HSP70 chaperonin. Taken together, the SUMO conjugation system appears to be a complex and functionally heterogeneous pathway for protein modification in plants with initial data indicating that one important function may be in stress protection and/or repair.Post-translational modifications of proteins play a critical role in most cellular processes through their unique ability to alter rapidly and reversibly the functions of preexisting proteins, multiprotein complexes, and intracellular structures. Although originally thought to be restricted to small molecules like phosphate and sugars, emerging data now show that several distinct types of polypeptide tags are important modifiers as well (1-4). These polypeptides become covalently attached to various intracellular targets via mechanistically similar ATPdependent reaction cascades involving activation (E1s) 1 and conjugation (E2s) enzymes. Sometimes an additional enzyme (E3s) also participates in target recognition and ligation. Ultimately, a protein conjugate is formed bearing the tag linked via an isopeptide bond between its C-terminal glycine and free lysyl ⑀-amino groups within the target. Depending on the tag and/or the target protein, the function, location, and/or half-life of the target can be affected. A family of tag-specific proteases also participates in each of the pathways. These proteases help generate the active form of the tag by removing extra residues that cap the C-terminal glycine of the polypeptide and/or are used to disassemble conjugates by cleaving the isopeptide bond between the tag and the target, thus releasing each in an unmodified form.The most pervasive and best understood tag is the 76-amino acid polypeptide ubiquitin (Ub) (5...

Identification and Molecular Properties of SUMO-Binding Proteins in Arabidopsis

“…In addition, overexpression of SUMO2 enhanced both the steady state levels of SUMO2 conjugates under normal growth conditions and showed subsequent heat shock-induced accumulation in Arabidopsis plants (Kurepa et al, 2003). To examine endogenous SUMO conjugates in independent HFAtSUMO1 transgenic Arabidopsis, we performed gel blot analyses using anti-His tag antiserum after heat shock treatment (Fig.…”

“…Specific interaction and sumoylation pattern between SUMO1 and MCM3 (At5G46280) protein Arabidopsis contains eight SUMO genes (Kurepa et al, 2003;Novatchkova et al, 2004). Only four of these (SUMO1, SUMO2, SUMO3, and SUMO5) are highly expressed, functional and probably of major importance, although the other SUMOs could be expressed in very specific cells or under specific conditions only.…”

“…Levels of SUMO1E2 conjugates are dramatically but transiently increased by various types of cellular stress, including exposure of the plants to heat shock, H 2 O 2 , ethanol, and canavanine, suggesting sumoylation as an important post-translational feature of the plant stress response. Indicating some specificity is the fact that levels and distribution of SUMO3 conjugates appear to be unaffected by any stress (Kurepa et al, 2003). Although our understanding of sumoylation remains limited in plants, the system appears to play an exceptionally important role in various plant environ-…”

Overexpression of SIZ1 enhances tolerance to cold and salt stresses and attenuates response to abscisic acid in Arabidopsis thaliana

“…According to the SUMO1/2-conjugation profile, the sumoylation of many proteins is stimulated by various environmental stresses (Catala et al 2007;Elrouby and Coupland 2010;Kurepa et al 2003;Miller et al 2010;Miura et al 2007b), resulting in the modification of the expression of many genes. SIZ1 is an important SUMO E3 ligase in plants and functions to modulate diverse signaling pathways at various levels.…”

“…SUM1 and SUM2 are functionally equivalent and are capable of forming poly-SUMO chains, whereas SUM3 does not perform this function. According to SUMO profiles, SUM1/2 and SUM3 have different specificities and possibly different targets (Castano-Miquel et al 2011;Kurepa et al 2003). SUMO is covalently conjugated to a target protein through a three-step cascade via E1, E2, and E3.…”