The effects of connexin phosphorylation on gap junctional communication

Gap junctions are specialized membrane domains composed of collections of channels that directly connect neighboring cells providing for the cell-to-cell diffusion of small molecules, including ions, amino acids, nucleotides, and second messengers. Vertebrate gap junctions are composed of proteins encoded by the "connexin" gene family. In most cases examined, connexins are modified post-translationally by phosphorylation. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the connexin "lifecycle", such as the trafficking, assembly/disassembly, degradation, as well as, the gating of gap junction channels. Since connexin43 (Cx43) is widely expressed in tissues and cell lines, we understand the most about how it is regulated, and thus, connexin43 phosphorylation is a major focus of this review. Recent reports utilizing new methodologies combined with the latest genome information have shown that activation of several kinases including protein kinase A, protein kinase C, p34(cdc2)/cyclin B kinase, casein kinase 1, mitogen-activated protein (MAP) kinase and pp60(src) kinase can lead to phosphorylation at 12 of the 21 serine and two of the six tyrosine residues in the C-terminal region of connexin43. In several cases, use of site-directed mutants of these sites have shown that these specific phosphorylation events can be linked to changes in gap junctional communication.