Crystal structures of the human SUMO‐2 protein at 1.6 Å and 1.2 Å resolution

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Abstract
The SUMO proteins are a class of small ubiquitin‐like modifiers. SUMO is attached to a specific lysine side chain on the target protein via an isopeptide bond with its C‐terminal glycine. There are at least four SUMO proteins in humans, which are involved in protein trafficking and targeting. A truncated human SUMO‐2 protein that contains residues 9–93 was expressed in Escherichia coli and crystallized in two different unit cells, with dimensions of a = b = 75.25 Å, c = 29.17 Å and a = b = 74.96 Å, c = 33.23 Å, both belonging to the rhombohedral space group R3. They diffracted X‐rays to 1.6 Å and 1.2 Å resolution, respectively. The structures were determined by molecular replacement using the yeast SMT3 protein as a search model. Subsequent refinements yielded R/Rfree values of 0.169/0.190 and 0.119/0.185, at 1.6 Å and 1.2 Å, respectively. The peptide folding of SUMO‐2 consists of a half‐open β‐barrel and two flanking α‐helices with secondary structural elements arranged as ββαββαβ in the sequence, identical to those of ubiquitin, SMT3 and SUMO‐1. Comparison of SUMO‐2 with SUMO‐1 showed a surface region near the C terminus with significantly different charge distributions. This may explain their distinct intracellular locations. In addition, crystal‐packing analysis suggests a possible trimeric assembly of the SUMO‐2 protein, of which the biological significance remains to be determined.