SAM68 is an mRNA-binding protein involved in mRNA processing in the nucleus that forms membraneless compartments called SAM68 Nuclear Bodies (SNBs). We found that intersectin 1 (ITSN1), a multidomain scaffold protein harboring five soluble SH3 domains, interacts with SAM68 proline-rich motifs (PRMs) surrounded by self-adhesive low complexity domains. While SAM68 is poorly soluble in vitro, the interaction of ITSN1 SH3 domains and mRNA with SAM68 enhances its solubility. In HeLa cells, the interaction between the first ITSN1 SH3 domain (SH3A) and P0, the N-terminal PRM of SAM68, induces the dissociation of SNBs. In addition, we reveal the ability of another SH3 domain (SH3D) of ITSN1 to bind to mRNAs. ITSN1 and mRNA may thus act in concert to promote SAM68 solubilization, consistent with the absence of mRNA in SNBs in cells. Together, these results support the notion of a specific chaperoning of PRM-rich SAM68 within nuclear ribonucleoprotein complexes by ITSN1 that may regulate the processing of a fraction of nuclear mRNAs, notably SAM68-controlled splicing events related to higher neuronal functions or cancer progression. This observation may also serve as a putative model of the interaction between other PRM-rich RBPs and signaling proteins harboring SH3 domains.

SAM68是一种mRNA结合蛋白，参与细胞核中的mRNA加工，形成无膜区室，称为SAM68核体（SNB）。我们发现intersectin 1（ITSN1），一个具有五个可溶性SH3域的多域支架蛋白，与被自粘性低复杂性域包围的SAM68富含脯氨酸的基序（PRM）相互作用。尽管SAM68在体外难溶，但ITSN1 SH3域和mRNA与SAM68的相互作用增强了其溶解度。在HeLa细胞中，第一个ITSN1 SH3域（SH3A）与P0（SAM68的N端PRM）之间的相互作用诱导了SNB的解离。此外，我们揭示了ITSN1的另一个SH3域（SH3D）结合mRNA的能力。因此，ITSN1和mRNA可能协同作用以促进SAM68增溶，这与细胞中SNB中不存在mRNA一致。一起，这些结果支持了ITSN1核糖核糖核蛋白复合物中富含PRM的SAM68特异性伴侣的构想，它可能调节部分核mRNA的加工，特别是与更高神经元功能或癌症进展有关的SAM68控制的剪接事件。该观察结果还可以用作其他富含PRM的RBP与带有SH3结构域的信号蛋白之间相互作用的推定模型。