A tyrosine-based sorting signal is involved in connexin43 stability and gap junction turnover.
The gap junction protein connexin43 is known to have a rapid turnover, involving degradation by both the proteasomal and lysosomal systems, but the structural features of connexin43 that govern these actions are not known. The connexin43 C-terminal sequence contains a proline-rich region corresponding to the consensus of a protein-protein interaction PY-motif (xPPxY), and an overlapping putative tyrosine-based sorting signal (Yxxphi; =hydrophobic), known to play a role in the intracellular trafficking of many membrane proteins. As both motifs may control turnover of connexin43, we used a combination of metabolic radiolabelling, immuno-precipitation and functional assays to determine the possible role of these motifs in controlling degradation of human connexin43 expressed in SKHep1 cells. Mutation V289D in the tyrosine-based sorting motif increased the steady-state pool of connexin43 by approximately 3.5-fold, while mutation P283L in the PY-motif produced a comparatively modest augmentation (1.7-fold). No additive effect was observed when the overlapping tyrosine was mutated. In pulse-chase experiments, the Y286A substitution increased the half-life of connexin43 from 2 to 6 hours, indicating that the increased steady-state levels reflected reduced protein degradation. Moreover, expression at the junctional membrane, as well as gap junction-mediated intercellular communication (GJC), were nearly abolished by lysosomal inhibitors and Brefeldin A in cells expressing wild-type connexin43, but were unaffected in the tyrosine mutant. These results provide strong evidence that the tyrosine-based motif of human connexin43 is a prime determinant controlling connexin43 stability, and consequently GJC, by targeting connexin43 for degradation in the endocytic/lysosomal compartment.
Institute of Pharmacology and Toxicology, University of Lausanne, 1005 Lausanne, Switzerland.
Marc A
MA
Thomas
Nathalie
N
Zosso
Isabelle
I
Scerri
Nicolas
N
Demaurex
Marc
M
Chanson
Olivier
O
Staub
D006528:Carcinoma, Hepatocellular; D045744:Cell Line, Tumor; D018031:Connexin 43 / Q000235:genetics / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism; D011992:Endosomes / Q000378:metabolism; D017629:Gap Junctions / Q000378:metabolism*; D006801:Humans; D008113:Liver Neoplasms; D008247:Lysosomes / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism; D016296:Mutagenesis / Q000502:physiology; D046988:Proteasome Endopeptidase Complex; D021382:Protein Sorting Signals / Q000502:physiology*; D021381:Protein Transport / Q000502:physiology; D014443:Tyrosine / Q000378:metabolism*
D018031:Connexin 43; D009097:Multienzyme Complexes; D021382:Protein Sorting Signals; D014443:Tyrosine; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex