TCBG Seminar

X-ray structures of the human Cx26 gap junction channel reveal an electrostatic mechanism for calcium-mediated ion selectivity and a model for channelopathies

Prof. Mark Yeager
Molecular Physiology and Biological Physics
University of Virginia School of Medicine
Charlottesville, VA

Monday, December 16, 2013
3:00 pm (CT)
3269 Beckman Institute


Dr. Yeager’s presentation will focus on recent progress related to the structure and function of gap junction channels. Hexameric connexin (Cx) hemichannels from adjacent cells dock end-to-end to form gap junction channels that mediate the passage of ions, second messengers, and metabolites, thereby providing intercellular signaling crucial in normal and pathological physiology. To explore the mechanism by which Ca2+ blocks ionic conductance during tissue injury, we solved X-ray crystal structures of a human gap junction channel with and without bound Ca2+. Three-dimensional crystals of recombinant Cx26 were grown using a new class of detergents designated facial amphiphiles (FAs), which have a cholate backbone with polar groups extending from one face and a short alkyl chain extending from the opposite face. FA-solubilized Cx26 crystallized into the H32 space group with two monomers in the asymmetric unit, and the crystals diffracted isotropically to 3.2 Å resolution. Crystals in the absence of Ca2+ were grown under similar conditions. A cryoEM map of Cx43 at 5.7-Å resolution [Fleishman et al., Mol. Cell 15: 879-888 (2004)] was used as a search model for molecular replacement. The Ca2+-bound and free structures were nearly identical, ruling out a large-scale steric mechanism for channel block. In both cases, the pore diameter was ~15 Å, sufficient for the passage of hydrated ions. The sites for Ca2+ coordination reside at the interface between adjacent subunits, near the entrance to the extracellular gap, accompanied by local conformational shifts of Ca2+-chelating residues. Molecular dynamics simulations and electrostatic calculations suggest that Ca2+ induces an electrostatic barrier to the passage of cations. A reduction of this electrostatic barrier provides an explanation for some Cx26 channelopathies that cause deafness.

Mark Yeager, M.D., Ph.D. is the Andrew P. Somlyo Professor and Chair of the Department of Molecular Physiology and Biological Physics, as well as a Professor of Cardiovascular Medicine, at the University of Virginia School of Medicine. Dr. Yeager received his undergraduate degree in chemistry from Carnegie-Mellon University, an M.Phil. and a Ph.D. degree from the Department of Molecular Biophysics and Biochemistry at Yale University and his medical degree from Yale University School of Medicine, where he was elected to Alpha Omega Alpha. His medicine residency, chief residency and specialty training in cardiology were performed at Stanford University Medical Center. In addition to support from the National Institutes of Health (NIH) and philanthropic grants, Dr. Yeager's contributions to biomedical research have been recognized with a Clinical Investigator Award from the NIH, an Established Investigator Award from the American Heart Association and a Clinical Scientist Award in Translational Research from the Burroughs Welcome Fund. Dr. Yeager directs a basic research laboratory examining the structure of macromolecular complexes using cryoEM, X-ray crystallography and molecular modeling. Research projects span (1) membrane proteins involved in cell to cell communication (gap junctions), transmembrane signaling (adenosine receptors, integrins), ion transport (Zn++ transporters, K+ channels), and water transport (aquaporins); (2) viruses responsible for significant human diseases (HIV, hepatitis B, hepatitis C, rotavirus, astrovirus); and (3) viruses used as model systems to understand mechanisms of pathogenesis (arenaviruses, reoviruses, nodaviruses, tetraviruses and sobemoviruses).

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