SUMO Protease SMT7 Modulates Ribosomal Protein L30 and Regulates Cell-Size Checkpoint Function.

Proliferating cells actively coordinate growth and cell division to ensure cell-size homeostasis; however, the underlying mechanism through which size is controlled is poorly understood. Defect in a SUMO protease protein, suppressor of 7 (SMT7), has been shown to reduce cell division number and increase cell size of the small-size mutant (), which contains a defective retinoblastoma tumor suppressor-related protein of Chlamydomonas (). Here we describe development of an in vitro SUMOylation system using Chlamydomonas components and use it to provide evidence that SMT7 is a SUMO protease. We further demonstrate that the SUMO protease activity is required for supernumerous mitotic divisions of the cells. In addition, we identified RIBOSOMAL PROTEIN L30 (RPL30) as a prime SMT7 target and demonstrated that its SUMOylation is an important modulator of cell division in cells. Loss of SMT7 caused elevated SUMOylated RPL30 levels. Importantly, overexpression of the translational fusion version of RPL30-SUMO4, which mimics elevation of the SUMOylated RPL30 protein in , caused a decrease in mitotic division and recapitulated the size-increasing phenotype of the cells. In summary, our study reveals a novel mechanism through which a SUMO protease regulates cell division in the mutant of Chlamydomonas and provides yet another important example of the role that protein SUMOylation can play in regulating key cellular processes, including cell division.