期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：11
页码：3547-3552
DOI：10.1073/pnas.1502291112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceTMEM16A is a calcium-activated chloride channel, meaning that it is a protein found at the surface of a variety of cells that permits chloride to enter when internal calcium levels rise. TMEM16A has been implicated in a variety of biological processes, including epithelial fluid secretion, smooth muscle contraction, neuron firing, and cancer cell proliferation. However, it is unclear how chloride ions are guided across the cell membrane by this protein. Here, we have modified and tested positively charged amino acids of TMEM16A and found four that modify chloride flux. We also tested potential blockers from a high-throughput screen and describe two that appear to block chloride's path, which should contribute to future studies attempting to further analyze TMEM16A's function. TMEM16A (transmembrane protein 16) (Anoctamin-1) forms a calcium-activated chloride channel (CaCC) that regulates a broad array of physiological properties in response to changes in intracellular calcium concentration. Although known to conduct anions according to the Eisenman type I selectivity sequence, the structural determinants of TMEM16A anion selectivity are not well-understood. Reasoning that the positive charges on basic residues are likely contributors to anion selectivity, we performed whole-cell recordings of mutants with alanine substitution for basic residues within the putative pore region and identified four residues on four different putative transmembrane segments that significantly increased the permeability of the larger halides and thiocyanate relative to that of chloride. Because TMEM16A permeation properties are known to shift with changes in intracellular calcium concentration, we further examined the calcium dependence of anion selectivity. We found that WT TMEM16A but not mutants with alanine substitution at those four basic residues exhibited a clear decline in the preference for larger anions as intracellular calcium was increased. Having implicated these residues as contributing to the TMEM16A pore, we scrutinized candidate small molecules from a high-throughput CaCC inhibitor screen to identify two compounds that act as pore blockers. Mutations of those four putative pore-lining basic residues significantly altered the IC50 of these compounds at positive voltages. These findings contribute to our understanding regarding anion permeation of TMEM16A CaCC and provide valuable pharmacological tools to probe the channel pore.
