Phosphorylated and non-phosphorylated connexin-32 molecules in gap junction plaques are protected against calpain proteolysis after phosphorylation by protein kinase C.

Introduction Gap junctions are formed by plaques of functionally assembled intercellular channels which are responsible for direct communication between the cytosols of adjacent cells in the form of ions and secondmessenger molecules. These channels are formed as two hexameric hemichannels of connexin (cnx) subunits that interlock head-on [ 1-61, Connexiris form an extensive family of well conserved proteins, which exhibit differential expression in a given tissue [7]. Thus, cnx-26 and cnx-32 are the two species expressed in adult liver. Assembly and disassembly of gap junctions occur very rapidly, and a half-life of 1-2 h for a connexin molecule has been measured (see [8] and references therein). This would be particularly true of tissues undergoing fast growth and morphogenesis, which would exhibit dramatic changes in the number of mitotic cells 191. Iittle is known about the pathways for assembly and disassembly of gap-junction channels. I t is evident, however, that such complex processes have to be highly regulated. We have proposed that proteolytic cleavage of connexins by calpain could be relevant for the concomitant and/or subsequent disassembly of gap junctions. Thus, phosphorylation of connexin by protein kinase C (PKC) may play a prominent regulatory role in this process, preventing unwanted connexin degradation [ 101. Conversely, connexin dephosphorylation could be a signal for the initiation of gap junction disassembly [ lo ] . We now describe that the protective effect mediated by phosphorylation by PKC is efficiently spread to neighbouring non-phosphorylated connexin molecules.