Regulation of connexin degradation as a mechanism to increase gap junctionassembly and function

Connexins, the integral membrane protein constituents of gap junctions, are degraded at a rate (t(1/2) = 1.5-5 h) much faster than most other cell sur face proteins. Although the turnover of connexins has been shown to be sens itive to inhibitors of either the lysosome or of the proteasome, how connex ins are targeted for degradation and whether this process can be regulated to affect intercellular communication is unknown. We show here that reducin g connexin degradation with inhibitors of the proteasome (but not with lyso somal blockers) is associated with a striking increase in gap junction asse mbly and intercellular dye transfer in cells inefficient in both processes under basal conditions. The effect of proteasome inhibitors on wild-type co nnexin stability, assembly, and function was mimicked by treatment of assem bly-inefficient cells with inhibitors of protein synthesis such as cyclohex imide. Sensitivity of connexin degradation to cycloheximide, but not to pro teasome inhibitors, was abolished when connexins were rendered structurally abnormal by perturbation of essential disulfide bonds or by mutation. Our findings provide the first evidence that intercellular communication can be up-regulated at the level of connexin turnover and that a short-lived prot ein may be required for conformationally mature connexins to become substra tes of proteasomal degradation.