Reorientation of aquaporin-1 topology during maturation in the endoplasmic reticulum

被引:107
作者
Lu, Y
Turnbull, IR
Bragin, A
Carveth, K
Verkman, AS
Skach, WR [1 ]
机构
[1] Oregon Hlth Sci Univ, Div Mol Med, Portland, OR 97201 USA
[2] Univ Penn, Dept Med, Philadelphia, PA 19104 USA
[3] Univ Calif San Francisco, Cardiovasc Res Unit, San Francisco, CA 94143 USA
关键词
D O I
10.1091/mbc.11.9.2973
中图分类号
Q2 [细胞生物学];
学科分类号
071009 ; 090102 ;
摘要
The topology of most eukaryotic polytopic membrane proteins is established cotranslationally in the endoplasmic reticulum (ER) through a series of coordinated translocation and membrane integration events. For the human aquaporin water channel AQP1, however, the initial four-segment-spanning topology at the ER membrane differs from the mature six-segment-spanning topology at the plasma membrane. Here we use epitope-tagged AQP1 constructs to follow the transmembrane (TM) orientation of key internal peptide loops in Xenopus oocyte and cell-free systems. This analysis revealed that AQP1 maturation in the ER involves a novel topological reorientation of three internal TM segments and two peptide loops. After the synthesis of TMs 4-6, TM3 underwent a 180-degree rotation in which TM3 C-terminal flanking residues were translocated from their initial cytosolic location into the ER lumen and N-terminal flanking residues underwent retrograde translocation from the ER lumen to the cytosol. These events convert TM3 from a type I to a type II topology and reposition TM2 and TM4 into transmembrane conformations consistent with the predicted six-segment-spanning AQP1 topology. AQP1 topological reorientation was also associated with maturation from a protease-sensitive conformation to a protease-resistant structure with water channel function. These studies demonstrate that initial protein topology established via cotranslational translocation events in the ER is dynamic and may be modified by subsequent steps of folding and/or maturation.
引用
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页码:2973 / 2985
页数:13
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