The first extracellular loop domain is a major determinant of charge selectivity in connexin46 channels

Intercellular channels formed of members of the gene family of connexins (C xs) vary from being substantially cation selective to being anion selective . We took advantage of the ability of Cx46 to function as an unapposed hemi channel to examine the basis of Cx charge selectivity. Previously we showed Cx46 hemichannels to be large pores that predominantly conduct cations and inwardly rectify in symmetric salts, properties suggesting selectivity is influenced by fixed negative charges located toward the extracellular end o f the pore. Here we demonstrate that high ionic strength solutions applied to the extracellular, but not the intracellular, side of Cx46 hemichannels substantially reduce the ratio of cation to anion permeability. Substitutio n of the first extracellular loop (E1) domain of Cx32, an anion-preferring Cx, reduces conductance, converts Cx46 from cation to anion preferring, and changes the I-V relation form inwardly to outwardly rectifying. These data suggest that fixed negative charges influencing selectivity in Cx46 are lo cated in E1 and are substantially reduced and/or are replaced with positive charges from the Cx32 E1 sequence. Extending studies to Cx46 cell-cell cha nnels, we show that they maintain a strong preference for cations, have a c onductance nearly that expected by the series addition of hemichannels, but lack rectification in symmetric salts. These properties are consistent wit h preservation of the fixed charge region in E1 of hemichannels, which upon docking, become symmetrically placed near the center of the cell-cell chan nel pore. Furthermore, heterotypic cell-cell channels formed by pairing Cx4 6 with Cx32 or Cx43 rectify in symmetric salts in accordance with the diffe rences in the charges we ascribed to E1. These data are consistent with cha rged residues in E1 facing the channel lumen and playing an important role in determining Cx channel conductance and selectivity.