BACKGROUND Sec4p is a small monomeric Ras-related GTP-binding protein (23 kDa) that regulates polarized exocytosis in S. cerevisiae. In this study we examine the structural effects of a conserved serine residue in the P-loop corresponding to G12 in Ras. RESULTS We show that the Sec4p residue serine 29 forms a hydrogen bond with the nucleotide. Mutations of this residue have a different impact than equivalent mutations in Ras and can form stable associations with the exchange factor allowing us to elucidate the structure of a complex of Sec4p bound to the exchange factor Sec2p representing an early stage of the exchange reaction. CONCLUSIONS Our structural investigation of the Sec4p-Sec2p complex reveals the role of the Sec2p coiled-coil domain in facilitating the fast kinetics of the exchange reaction. For Ras-family GTPases, single point mutations that impact the signaling state of the molecule have been well described however less structural information is available for equivalent mutations in the case of Rab proteins. Understanding the structural properties of mutants such as the one described here, provides useful insights into unique aspects of Rab GTPase function.

中文翻译：

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Rab GTPase Sec4p激活的分子机制的新见解。

背景Sec4p是一种小单体Ras相关的GTP结合蛋白（23 kDa），可调节酿酒酵母中的极化胞吐作用。在这项研究中，我们检查了对应于Ras中G12的P环中保守丝氨酸残基的结构效应。结果我们显示，Sec4p残基丝氨酸29与核苷酸形成氢键。该残基的突变与Ras中的等效突变具有不同的影响，并且可以与交换因子形成稳定的关联，从而使我们能够阐明与交换因子Sec2p结合的Sec4p复合物的结构，该复合物代表交换反应的早期阶段。结论我们对Sec4p-Sec2p复合物的结构研究揭示了Sec2p卷曲螺旋结构域在促进交换反应快速动力学中的作用。对于Ras家族的GTPases，已经充分描述了影响分子信号传导状态的单点突变，但是在Rab蛋白的情况下，较少的结构信息可用于等效突变。了解突变体的结构特性（例如此处所述的突变体）可提供有关Rab GTPase功能独特方面的有用见解。