Biogenesis and Folding of Aquaporin Water Channels in the Endoplasmic Reticulum

Abstract

The transmembrane topology of most eukaryotic polytopic membrane proteins is established in the endoplasmic reticulum (ER). This process involves multiple translocation and membrane integration events that are facilitaed coincident with protein synthesis by the Sec6l translocation machinery. Little is known, however, regarding the molecular mechanisms by which multiple transmembrane (TM) helices and peptide loops are precisely oriented into the ER membrane, ER lumen, and cytosol. Aquaporins have provided a unique opportunity to study this phenomenon because related family members acquire their topology through distinctly different mechanisms. The six TM segments of AQP4 are established sequentially and cotranslationally as the nascent polypeptide emerges from the ribosome (Shi et al., 1995). In contrast, the initial topology of AQP1 at the ER membrane contains only four TM segments (Skach et al., 1994). To better understand aquaporin folding and maturation, we have examined: 1) the process by which AQP1 acquires its mature, six-spanning orientation, and 2) the structural basis for different AQP1 and AQP4 folding pathways.