Babic I, Cherry E, Fujita D J
Department of Biochemistry and Molecular Biology, Southern Alberta Cancer Research Institute, University of Calgary, Calgary, Alberta, Canada.
Oncogene. 2006 Aug 17;25(36):4955-64. doi: 10.1038/sj.onc.1209504. Epub 2006 Mar 27.
Sam68 (Src associated in mitosis; 68 kDa) is an RNA-binding protein and substrate of Src family kinases. It is thought to play a role in cell cycle progression. Overexpression of Sam68 in fibroblasts was reported to have two separable functions dependent on its ability to bind RNA--cell cycle arrest or the induction of apoptosis. Post-translational modification with SUMO (small ubiquitin-like modifier) is common to many transcription factors and can regulate protein localization, stability and function. Here we show Sam68 to be modified by SUMO, and demonstrate that the SUMO E3 ligase (PIAS1) (protein inhibitor of activated STAT1) can enhance Sam68 sumoylation. Lysine 96, the first lysine in the amino-terminal region of Sam68, was found to be the major SUMO acceptor site. Mutation of the SUMO acceptor lysine to arginine enhanced the ability of Sam68 to induce apoptosis but inhibited its ability to act as a transcriptional inhibitor of cyclin D1 expression. A SUMO-1 Sam68 fusion protein, on the other hand, inhibited the ability of Sam68 to induce apoptosis but was a strong repressor of cyclin D1 expression. Thus, SUMO may be an important regulator of Sam68 function in cell cycle progression.
Sam68（有丝分裂相关的Src；68千道尔顿）是一种RNA结合蛋白，也是Src家族激酶的底物。它被认为在细胞周期进程中发挥作用。据报道，在成纤维细胞中过表达Sam68具有两种可分离的功能，这取决于其结合RNA的能力——细胞周期停滞或诱导细胞凋亡。用小泛素样修饰物（SUMO）进行的翻译后修饰在许多转录因子中很常见，并且可以调节蛋白质的定位、稳定性和功能。在这里，我们展示了Sam68被SUMO修饰，并证明SUMO E3连接酶（PIAS1）（活化STAT1的蛋白抑制剂）可以增强Sam68的SUMO化。发现Sam68氨基末端区域的第一个赖氨酸残基赖氨酸96是主要的SUMO接受位点。将SUMO接受赖氨酸突变为精氨酸增强了Sam68诱导细胞凋亡的能力，但抑制了其作为细胞周期蛋白D1表达转录抑制剂的能力。另一方面，SUMO-1 Sam68融合蛋白抑制了Sam68诱导细胞凋亡的能力，但却是细胞周期蛋白D1表达的强抑制剂。因此，SUMO可能是细胞周期进程中Sam68功能的重要调节因子。
