Selectivity of Connexin43 and Connexin40 Comprised Gap Junctions

Gap junctions are aggregates of intercellular channels each formed of protein subunits termed connexins (Cx). Recently published data show that junctional dye permeability relative to conductance (permselectivity) varies across several orders of magnitude for Cx43 junctions, suggesting variable selectivity of the comprising Cx43 channels. Logical candidates for this variable selectivity are variability in charge or size selectivity. Consequently, junctional charge and size selectivities were determined in the current study by simultaneous measurement of junctional permeance to dyes of differing size or charge.The results show that for a number of dyes differing in size, charge, chemical composition, and structure the primary determinant for selectivity through Cx43 gap junctions was the size of the dye permeant with this selectivity showing essentially no variability beyond that seen between incompletely divided cells, presumably representing the variability inherent to the measurement. As such, selectivity of dye-permeable Cx43 channels is well described by the physical dimensions of the channel pore acting essentially as a simple molecular sieve. The seemingly disparate dye selectivity and permselectivity results can be reconciled by the variable presence of a dye-impermeable but electrically conductive channel conformation for Cx43 channels, affording a possible mechanism for independent regulation of diffusion of larger molecules versus electrical conductance to smaller ions.Cx40 junctions, known to be cation selective, also showed minimal variability in charge selectivity indicating that Cx40 charge selectivity is also an essentially fixed parameter. Co-expression of Cx40 and Cx43 lead to charge selectivities ranging from Cx43 to Cx40 with an average intermediate between the two. Activation of PKC leads to an increase in cationic selectivity of Cx40/Cx43 composed junctions by specifically reducing permeability through non-selective Cx43 channels favoring permeation through cation-selective Cx40 channels, allowing for junctional charge selectivity regulation.The combined data suggest that selectivity properties for dye permeable channels composed of Cx43 or Cx40 are essentially fixed parameters of the channel pore. Only upon co-expression of these connexins is significant variability in selectivity seen. The differential effects of PKC-mediated phosphorylation on permeability of Cx43 and Cx40 channels then allows for regulation of junctional charge selectivity but only in cells expressing both connexins