Proliferating cells actively coordinate growth and cell division to ensure cell-size homeostasis; however, the underlying mechanism through which size is controlled is still poorly understood. Defect in a SUMO protease protein, SMT7, has been shown to reduce cell division number and increase cell size of the small-size mutant mat3-4, which contains a defective Chlamydomonas retinoblastoma tumor suppressor-related protein. Here we describe development of an in vitro SUMOylation system using Chlamydomonas components and use it to provide evidence that SMT7 is a bona fide SUMO protease. We further demonstrate that the SUMO protease activity is required for supernumerous mitotic divisions of the mat3-4 cells. In addition, we identified ribosomal protein L30 (RpL30) as a prime SMT7 target and discovered that its SUMOylation is important to modulate cell division in mat3-4 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 mat3-4 caused decrease in mitotic division and recapitulated the size-increasing phenotype of the smt7-1 mat3-4 cells. In summary, our studies reveal a novel mechanism through which a SUMO protease regulates cell division in the mat3-4 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.

增殖细胞积极协调生长和细胞分裂，以确保细胞大小的稳态。但是，对于控制大小的基本机制仍然知之甚少。SUMO蛋白酶蛋白SMT7中的缺陷已显示出可减少细胞突变数并增加小型突变体mat3-4的细胞大小，该突变体包含有缺陷的衣原体视网膜母细胞瘤肿瘤抑制相关蛋白。在这里，我们描述了使用衣藻成分的体外SUMOylation系统的开发，并使用它来提供SMT7是真正的SUMO蛋白酶的证据。我们进一步证明，SUMO蛋白酶活性是mat3-4细胞大量有丝分裂所必需的。此外，我们确定了核糖体蛋白L30（RpL30）为主要的SMT7靶标，并发现其SUMOylation对调节mat3-4细胞中的细胞分裂很重要。SMT7的丢失导致SUMO化RpL30水平升高。重要的是，RpL30-SUMO4的翻译融合版本的过表达，类似于mat3-4中SUMO化的RpL30蛋白的升高，导致有丝分裂减少，并概括了smt7-1 mat3-4细胞的大小增加表型。总而言之，我们的研究揭示了一种新的机制，SUMO蛋白酶可通过该机制调节衣原体的mat3-4突变体中的细胞分裂，并提供蛋白质SUMOylation在调节关键细胞过程（包括细胞分裂）中所起的作用的另一个重要实例。RpL30-SUMO4的翻译融合版本的过表达，类似于mat3-4中SUMO酰化RpL30蛋白的升高，导致有丝分裂减少，并概括了smt7-1 mat3-4细胞的大小增加表型。总而言之，我们的研究揭示了一种新的机制，SUMO蛋白酶可通过该机制调节衣原体的mat3-4突变体中的细胞分裂，并提供蛋白质SUMOylation在调节关键细胞过程（包括细胞分裂）中所起的作用的另一个重要实例。RpL30-SUMO4的翻译融合版本的过表达，类似于mat3-4中SUMO酰化RpL30蛋白的升高，导致有丝分裂减少，并概括了smt7-1 mat3-4细胞的大小增加表型。总而言之，我们的研究揭示了一种新的机制，SUMO蛋白酶可通过该机制调节衣原体的mat3-4突变体中的细胞分裂，并提供蛋白质SUMOylation在调节关键细胞过程（包括细胞分裂）中所起的作用的另一个重要实例。