Intracellular transport, assembly, and degradation of wild-type and disease-linked mutant gap junction proteins

More than 130 different mutations in the gap junction integral plasma membr ane protein connexin32 (Cx32) have been linked to the human peripheral neur opathy X-linked Charcot-Marie-Tooth disease (CMTX). How these various mutan ts are processed by the cell and the mechanism(s) by which they cause CMTX are unknown. To address these issues, we have studied the intracellular tra nsport, assembly, and degradation of three CMTX-linked Cx32 mutants stably expressed in PC12 cells. Each mutant had a distinct fate: E208K Cx32 appear ed to be retained in the endoplasmic reticulum (ER), whereas both the E186K and R142W mutants were transported to perinuclear compartments from which they trafficked either to lysosomes (R142W Cx32) or back to the ER (E186K C x32). Despite these differences, each mutant was soluble in nonionic deterg ent but unable to assemble into homomeric connexons. Degradation of both mu tant and wild-type connexins was rapid (t(1/2) < 3 h) and took place at lea st in part in the ER by a process sensitive to proteasome inhibitors. The m utants studied are therefore unlikely to cause disease by accumulating in d egradation-resistant aggregates but instead are efficiently cleared from th e cell by quality control processes that prevent abnormal connexin molecule s from traversing the secretory pathway.