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

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 inEscherichia coliand 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 groupR3. 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 yieldedR/ 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-openβ-barrel and two flankingα-helices with secondary structural elements arranged asββαββαβ 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. [ABSTRACT FROM AUTHOR]