RUB和马普研究所的研究人员揭示了细胞转运调控蛋白的新作用机制，他们发现核心开关蛋白Rab的活性受到其互作蛋白两个指状结构的调解，文章发表在美国国家科学院院刊PNAS杂志上。调控细胞生长的Ras蛋白只受到一个指状结构的调解，而Rab则需要两个指状结构，研究人员由此解密了GTPases家族的新作用机制。
GTPases是我们体内一类重要的小分子，Ras和Rab都是其中的一员。Rab蛋白控制不同细胞区域之间的多种运输过程，如果运输系统被破坏就会发生肥胖症等疾病。Rab和Ras蛋白作用相似，都类似于一个开关。当开关开启时高能分子GTP结合，而开关关闭时结合的是低能分子GDP。RabGAP蛋白负责催化GTP的水解，在这一过程中GTP分解为GDP和一个磷酸基团。现在，研究人员以最高原子分辨率观察并记录了上述过程。
动态机制
研究人员通过X射线结构分析，首次确定了RabGAP蛋白复合体的空间结构。研究显示，这一复合体中存在两个指状结构，一个精氨酸指一个谷氨酰胺指。人们已知精氨酸指结构也存在于Ras复合体中，而谷氨酰胺指的存在则出人意料。研究显示，RabGAP的两个指状结构插入Rab蛋白的GTP结合区域，使GTP水解加速了五个数量级。
研究人员用傅里叶变换红外光谱技术FTIR实时观察了这一动态过程，与X射线结构分析相比，傅里叶变换红外光谱技术不仅可以提供反应的快照还能够捕捉到整个动态过程。研究显示，正是两个催化性指状结构同时插入GTP结合区域，促使了GTP的水解。
医疗价值
这项研究所用的蛋白是Rab1b和RabGAP TBC1D20，不过研究人员指出其他Rab和RabGAP与他们选取的蛋白很相似，并由此推测其他Rab和RabGAP的活性机制也是通过两个指状结构实现的。GTPases家族的小分子发生突变，可能释放出不可控的生长信号，促使肿瘤形成。研究人员认为若能开发出模拟双指机制的小分子，就可以对这一过程进行调控，从而治疗癌症。(生物谷Bioon.com)
doi: 10.1073/pnas.1214431110
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Catalytic mechanism of a mammalian Rab·RabGAP complex in atomic detail
Konstantin Gavriljuka,1, Emerich-Mihai Gazdagb,1, Aymelt Itzenb,2, Carsten Köttinga, Roger S. Goodyb, and Klaus Gerwerta,3
Rab GTPases, key regulators of vesicular transport, hydrolyze GTP very slowly unless assisted by Rab GTPase-activating proteins (RabGAPs). Dysfunction of RabGAPs is involved in many diseases. By combining X-ray structure analysis and time-resolved FTIR spectroscopy we reveal here the detailed molecular reaction mechanism of a complex between human Rab and RabGAP at the highest possible spatiotemporal resolution and in atomic detail. A glutamine residue of Rab proteins (cis-glutamine) that is essential for intrinsic activity is less important in the GAP-activated reaction. During generation of the RabGAP·Rab:GTP complex, there is a rapid conformational change in which the cis-glutamine is replaced by a glutamine from RabGAP (trans-glutamine); this differs from the RasGAP mechanism, where the cis-glutamine is also important for GAP catalysis. However, as in the case of Ras, a trans-arginine is also recruited to complete the active center during this conformational change. In contrast to the RasGAP mechanism, an accumulation of a state in which phosphate is bound is not observed, and bond breakage is the rate-limiting step. The movement of trans-glutamine and trans-arginine into the catalytic site and bond breakage during hydrolysis are monitored in real time. The combination of X-ray structure analysis and time-resolved FTIR spectroscopy provides detailed insight in the catalysis of human Rab GTPases
