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：2016
卷号：113
期号：49
页码：E7986-E7995
DOI：10.1073/pnas.1609378113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceConnexin channels are ubiquitous, providing pathways for movement of molecules between cells (junctional channels) and for release of molecular effectors into the extracellular environment (plasma membrane hemichannels). To maintain an adequate permeability barrier, hemichannels are tightly regulated by normal extracellular Ca2+ to be closed under most conditions. Connexin mutations that disrupt hemichannel regulation by Ca2+ cause human pathologies due to aberrantly open hemichannels. Here we elucidate molecular mechanisms of gating by Ca2+ in hemichannels: Ca2+ binding causes a reorganization of specific interactions within the connexin protein that lead to a closed channel. Further, we show that the actual "gate" is deeper into the pore from where Ca2+ binds. The interactions involved are conserved across connexins, pointing to a general mechanism. Aberrant opening of nonjunctional connexin hemichannels at the plasma membrane is associated with many diseases, including ischemia and muscular dystrophy. Proper control of hemichannel opening is essential to maintain cell viability and is achieved by physiological levels of extracellular Ca2+, which drastically reduce hemichannel activity. Here we examined the role of conserved charged residues that form electrostatic networks near the extracellular entrance of the connexin pore, a region thought to be involved in gating rearrangements of hemichannels. Molecular dynamics simulations indicate discrete sites for Ca2+ interaction and consequent disruption of salt bridges in the open hemichannels. Experimentally, we found that disruption of these salt bridges by mutations facilitates hemichannel closing. Two negatively charged residues in these networks are putative Ca2+ binding sites, forming a Ca2+-gating ring near the extracellular entrance of the pore. Accessibility studies showed that this Ca2+-bound gating ring does not prevent access of ions or small molecules to positions deeper into the pore, indicating that the physical gate is below the Ca2+-gating ring. We conclude that intra- and intersubunit electrostatic networks at the extracellular entrance of the hemichannel pore play critical roles in hemichannel gating reactions and are tightly controlled by extracellular Ca2+. Our findings provide a general mechanism for Ca2+ gating among different connexin hemichannel isoforms.
