Basolateral membrane expression of a K+ channel, Kir 2.3, is directed by a cytoplasmic COOH-terminal domain.
The inwardly rectifying potassium channel Kir 2.3 is specifically targeted and expressed on the basolateral membrane of certain renal epithelial cells. In the present study, the structural basis for polarized targeting was elucidated. Deletion of a unique COOH-terminal domain produced channels that were mistargeted to the apical membrane, consistent with the removal of a basolateral membrane-sorting signal. By characterizing a series of progressively smaller truncation mutants, an essential targeting signal was defined (residues 431-442) within a domain that juxtaposes or overlaps with a type I PDZ binding motif (442). Fusion of the COOH-terminal structure onto CD4 was sufficient to change a random membrane-trafficking and expression pattern into a basolateral membrane one. Using metabolic labeling and pulse-chase and surface immunoprecipitation, we found that CD4-Kir2.3 COOH-terminal chimeras were rapidly and directly targeted to the basolateral membrane, consistent with a sorting signal that is processed in the biosynthetic pathway. Collectively, the data indicate that the basolateral sorting determinant in Kir 2.3 is composed of a unique arrangement of trafficking motifs, containing tandem, conceivably overlapping, biosynthetic targeting and PDZ-based signals. The previously unrecognized domain corresponds to a highly degenerate structure within the Kir channel family, raising the possibility that the extreme COOH terminus of Kir channels may differentially coordinate membrane targeting of different channel isoforms.
Le Maout
S
S
Department de Biologie Cellulaire et Moléculaire, Commissariat Energie Atomique, Saclay, Gif-Yvette 91191, France.
Welling
P A
PA
Brejon
M
M
Olsen
O
O
Merot
J
J
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D004285:Dogs; D007668:Kidney / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D015221:Potassium Channels / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D021382:Protein Sorting Signals / Q000235:genetics; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D014162:Transfection; D020798:Two-Hybrid System Techniques; D014443:Tyrosine / Q000737:chemistry
D015221:Potassium Channels; D021382:Protein Sorting Signals; D011993:Recombinant Fusion Proteins; D014443:Tyrosine