Is the junctional uncoupling elicited in rat ventricular myocytes by some dephosphorylation treatments due to changes in the phosphorylation status of Cx43?

Gap junctions, specialized membrane structures that mediate cell-to-cell communication in almost all animal tissues, are composed of channel-forming integral membrane proteins termed connexins. Most of them, particularly connexin43 (Cx43), the most ubiquitous connexin, the major connexin present in cardiac myocytes, are phosphoproteins. Connexin phosphorylation has been thought to regulate gap junctional protein trafficking, gap junction assembly, channel gating, and turnover. Some connexins, including Cx43, show mobility shifts in gel electrophoresis when cells are exposed to phosphorylating or dephosphorylating treatments. However, after exposure of rat cardiac myocytes to different uncoupling dephosphorylating agents such as H7 or butanedione monoxime, no modification in the Cx43 phosphorylation profile was generally observed. The lack of direct correlation between the inhibition of cell-to-cell communication and changes in the phosphorylation pattern of Cx43 or, conversely, modifications of the latter without modifications of the intercellular coupling degree, suggest that the functional state of junctional channels might rather be determined by regulatory proteins associated with Cx43. The modulation of the activity of junctional channels by protein phosphorylation/dephosphorylation processes very likely requires (as for several other membrane channels) the formation of a multiprotein complex, where pore-forming subunits bind to auxiliary proteins (e.g. scaffolding proteins, enzymes, cytoskeleton elements) that play essential roles in channel localization and activity. Such regulatory proteins, behaving as targets for phosphorylation/dephosphorylation catalysers, might in particular control the open probability of junctional channels. A schematic illustration of the regulation of Cx43-made channels by protein phosphorylation involving a partner phosphoprotein is proposed.