A SUMO ligase AtMMS21 regulates activity of the 26S proteasome in root development.

The 26S proteasome is a multi-subunit protease controlling most of the cytosolic and nuclear protein turnover, regulating many cellular events in eukaryotes. However, functional modification on this complex remains unclear. Here, we showed a novel mechanism that a SUMO ligase AtMMS21 regulates activity of the 26S proteasome in root development of Arabidopsis. Our in vitro and in vivo data supported that AtMMS21 interacts with RPT2a, a subunit of the 26S proteasome. The mutants of AtMMS21 and RPT2a display similar developmental defect of roots, suggesting their association in this process. In addition, RPT2a is modified by SUMO3, potentially related to AtMMS21. During development, the activity of the 26S proteasome is lower in both mutants of AtMMS21 and RPT2a, compared with that of wild type. Furthermore, the protein level but not the RNA level of RPT2a is decreased in the absence of AtMMS21, implying stability regulation of the proteasome complex through the AtMMS21-RPT2a interaction. Taken together, the current study would improve our understanding on the regulatory mechanism of the 26S proteasome via protein modification in root development.