Molecular Mechanism of Cation Channel Selectivity

描述(申请人提供)：离子在生物膜上的转移是神经兴奋、肌肉细胞收缩、信号转导和激素分泌等生理过程的核心。离子通道发挥着至关重要的作用，它在膜内提供了一个通道，即离子传导孔，允许特定的离子沿其电化学梯度向下移动。在人类中，离子通道几乎存在于所有具有各种生理必需功能的组织中。由于它们在人体中的普遍存在和重要性，经络功能障碍经常被归咎于一系列人类疾病。选择特定离子物种的能力称为离子选择性，是定义离子通道功能的基本属性。四聚阳离子通道是包括K+、Ca~(2+)、Na~+和环核苷酸门控(CNG)通道在内的最大的离子通道家族，其选择性通常是部分离子通道孔内独特的结构和化学环境--选择性过滤器的直接结果，不同通道的选择性过滤器是不同的。随着离子通道的遗传、生化和电生理分析的发展，以及最近从KCSA K+通道结构的开创性工作开始，以及随后对其他K+选择性通道的研究，我们对这类通道中控制离子选择性的分子细节的理解已经取得了很大进展。虽然这些结构研究提供了一个直观的K+通道选择性过滤的可视化，但决定K+选择性的因素仍然处于激烈的争论中。此外，其他四聚阳离子通道的结构信息很少，其离子选择性的分子基础尚不清楚。我们研究的总体目标是了解四聚阳离子通道中离子选择性的结构基础。利用几种模型蛋白质的极高分辨率晶体结构来描述选择性和非选择性通道的离子传导孔，我们的目的是阐明两组生理必需的阳离子通道中离子选择性的基本原理。一种是非选择性、钙离子通透性的环核苷酸门控(CNG)通道，以蜡状芽孢杆菌的NAK及其类似CNG的嵌合体为模型系统；另一种是K+选择性通道，以K+选择性NAK突变体(NaK2K)和来自自养甲烷杆菌的MthK+通道为模型系统。建议的研究将采用蛋白质结晶学、电生理学和蛋白质化学合成相结合的方法。 与公共健康相关：离子通道控制跨细胞膜的离子转移，这是神经兴奋、肌肉细胞收缩、信号转导和激素分泌的核心生物过程。毫不奇怪，鉴于它们在我们许多生物功能中的核心作用，这些通道的缺陷会导致各种致残的人类疾病。例如，某些形式的癫痫、耳聋、共济失调和心律失常等疾病直接可归因于离子通道功能障碍。

DESCRIPTION (provided by applicant): Transfer of ions across biological membranes is central to physiological processes like nerve excitation, muscle cell contraction, signal transduction and hormone secretion. Ion channels play a vital role by providing a passageway, the ion conduction pore, within membranes to allow specific ions to traverse down their electrochemical gradient. In humans, ion channels are found in nearly all tissues serving a variety of physiologically essential functions. Because of their prevalence and importance in the human body, dysfunction of a channel is often to blame for a wide range of human pathologies. The ability to select for specific ionic species is known as ion selectivity and is a fundamental property defining ion channel function. In tetrameric cation channels, which comprise the single largest family of ion channels including the K+, Ca2+, Na+, and cyclic nucleotide-gated (CNG) channels, selectivity is usually a direct consequence of the unique structural and chemical environment within part of the ion channel pore, the selectivity filter, which is distinct among different channels. Our understanding of the molecular details governing ion selectivity in this group of channels has come a long way with the advancement of genetic, biochemical, and electrophysiological analysis of ion channels and, more recently, the structural characterization of several members beginning with the ground breaking work of the KcsA K+ channel structure and subsequent work on other K+ selective channels. Although these structural studies offer a direct visualization of the selectivity filter of K+ channels, the determinant factors contributingto K+ selectivity are still under heated debate. Moreover, there is little structural information available for other tetrameric cation channels and the molecular basis for their ion selectivity remains unclear. The overall goal of our research is to understand the structural basis of ion selectivity in tetrameric cation channels. Taking advantage of the extremely high resolution crystal structures of several model proteins representing the ion conduction pores of both selective and non-selective channels, we aim to elucidate basic principles of ion selectivity in two groups of physiologically essential cation channels. One is the non-selective, Ca2+ permeable cyclic nucleotide-gated (CNG) channels, using NaK from Bacillus cereus and its CNG- mimicking chimeras as model systems; the other is K+ selective channels, using a K+ selective NaK mutant (NaK2K) and the MthK K+ channel from Methanobacterium thermoautotrophicum as model systems. A combined approach of protein crystallography, electrophysiology and protein chemical synthesis will be employed in the proposed studies PUBLIC HEALTH RELEVANCE: Ion channels control the transfer of ions across cell membranes, a biological process central to nerve excitation, muscle cell contraction, signal transduction, and hormone secretion. Not surprisingly, given their central role in much of our biological functioning, defects in these channels lead to a variety of disabling human disorders. Diseases such as certain forms of seizures, deafness, ataxia, and cardiac arrhythmias, for instance, are directly attributable to ion channel dysfunction.