Glycosylation is important for cell surface expression of the water channel aquaporin-2 but is not essential for tetramerization in the endoplasmic reticulum.
Aquaporin-2 (AQP2) is a pore-forming protein that is required for regulated reabsorption of water from urine. Mutations in AQP2 lead to nephrogenic diabetes insipidus, a disorder in which functional AQP2 is not expressed on the apical cell surface of kidney collecting duct principal cells. The mechanisms and pathways directing AQP2 from the endoplasmic reticulum to the Golgi complex and beyond have not been defined. We found that approximately 25% of newly synthesized AQP2 is glycosylated. Nonglycosylated and complex-glycosylated wild-type AQP2 are stable proteins with a half-life of 6-12 h and are both detectable on the cell surface. We show that AQP2 forms tetramers in the endoplasmic reticulum during or very early after synthesis and reaches the Golgi complex in 1-1.5 h. We also report that glycosylation is neither essential for tetramerization nor for transport from the endoplasmic reticulum to the Golgi complex. Instead, the N-linked glycan is important for exit from the Golgi complex and sorting of AQP2 to the plasma membrane. These results are important for understanding the molecular mechanisms responsible for the intracellular retention of AQP2 in nephrogenic diabetes insipidus.
Department of Cell Biology, University Medical Center Utrecht, Utrecht 3584 CX, The Netherlands.
Giel
G
Hendriks
Marco
M
Koudijs
Bas W M
BW
van Balkom
Viola
V
Oorschot
Judith
J
Klumperman
Peter M T
PM
Deen
Peter
P
van der Sluijs
D000818:Animals; D051399:Aquaporin 2; D051403:Aquaporin 6; D020346:Aquaporins / Q000737:chemistry / Q000378:metabolism*; D002451:Cell Compartmentation; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D019281:Dimerization; D004285:Dogs; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006031:Glycosylation; D006801:Humans; D011487:Protein Conformation; D021381:Protein Transport
C496023:AQP2 protein, human; D051399:Aquaporin 2; D051403:Aquaporin 6; D020346:Aquaporins