A stress-dependent SUMO4 sumoylation of its substrate proteins.

Here we performed studies to demonstrate SUMO4 maturation process. Unlike other SUMO proteins, cells under physiological condition mediate a rapid degradation for SUMO4. However, when cells under stressed condition, SUMO4 can be matured by the stress-induced endogenous hydrolase and be able to covalently conjugate to its substrate proteins. Furthermore, we failed to obtain evidence supporting a role for proline-90 unique to SUMO4 in its activation and functionality. Both wild-type SUMO4 and SUMO4-P90Q can be hydrolyzed by the stressed RAW264.7 cell lysates, and no significant functional difference between SUMO4, SUMO4-P90Q, and SUMO4-GG (matured form) was observed as determined by luciferase assay. However, the C-terminal di-glycine motif, a prerequisite for sumoylation, is necessary for SUMO4 to exert its functional activity. These data not only confirmed our previous published data, but also provided additional evidence suggesting a role for SUMO4 sumoylation in the regulation of intracellular stress.