PDF(Pubmed)
Abstract：
Two-pore channels are pathophysiologically important Na+- and Ca2+-permeable channels expressed in lysosomes and other acidic organelles. Unlike most other ion channels, their permeability is malleable and ligand-tuned such that when gated by the signaling lipid PI(3,5)P2, they are more Na+-selective than when gated by the Ca2+ mobilizing messenger nicotinic acid adenine dinucleotide phosphate. However, the structural basis that underlies such plasticity and single-channel behavior more generally remains poorly understood. A recent Cryo-electron microscopy (cryo-EM) structure of TPC2 bound to PI(3,5)P2 in a proposed open-channel conformation provided an opportunity to address this via molecular dynamics (MD) simulation. To our surprise, simulations designed to compute conductance through this structure revealed almost no Na+ permeation events even at very high transmembrane voltages. However further MD simulations identified a spontaneous transition to a dramatically different conformation of the selectivity filter that involved expansion and a flip in the orientation of two core asparagine residues. This alternative filter conformation was remarkably stable and allowed Na+ to flow through the channel leading to a conductance estimate that was in very good agreement with direct single-channel measurements. Furthermore, this conformation was more permeable for Na+ over Ca2+. Our results have important ramifications not just for understanding the control of ion selectivity in TPC2 channels but also more broadly in terms of how ion channels discriminate ions.
摘要：
双孔通道是在溶酶体和其他酸性细胞器中表达的病理生理上重要的Na和Ca2渗透通道。与大多数其他离子通道不同，它们的通透性是可延展的和配体调节的,使得当通过信号脂质PI(3,5)P2门控时,它们比通过Ca2+动员信使烟酸腺嘌呤二核苷酸磷酸门控时更具Na+选择性。然而,作为这种可塑性和单通道行为基础的结构基础通常仍然知之甚少。最近提出的开放通道构象中与PI（3,5）P2结合的TPC2的低温电子显微镜（cryo-EM）结构提供了通过分子动力学（MD）模拟解决这一问题的机会。令我们惊讶的是，设计用于计算通过该结构的电导的模拟显示,即使在非常高的跨膜电压下,也几乎没有Na+渗透事件。然而，进一步的MD模拟确定了向选择性过滤器的明显不同构象的自发转变，该选择性过滤器涉及两个核心天冬酰胺残基的膨胀和取向的翻转。这种替代的过滤器构象非常稳定，并且允许Na流过通道，从而导致电导率估算与直接单通道测量非常吻合。此外，这种构象对Na+的渗透性高于Ca2+。我们的结果不仅对于理解TPC2通道中离子选择性的控制，而且在离子通道如何区分离子方面也具有重要意义。
