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.

Polytopic protein topology is established in the endoplasmic reticulum (ER) by sequence determinants encoded throughout the nascent polypeptide. Here we characterize 12 topogenic determinants in the cystic fibrosis transmembrane conductance regulator, and identify a novel mechanism by which a charged residue is positioned within the plane of the lipid bilayer. During cystic fibrosis transmembrane conductance regulator biogenesis, topology of the C-terminal transmembrane domain (TMs 7-12) is directed by alternating signal (TMs 7, 9, and 11) and stop transfer (TMs 8, 10, and 12) sequences. Unlike conventional stop transfer sequences, however, TM8 is unable to independently terminate translocation due to the presence of a single charged residue, Asp 924 , within the TM segment. Instead, TM8 stop transfer activity is specifically dependent on TM7, which functions both to initiate translocation and to compensate for the charged residue within TM8. Moreover, even in the presence of TM7, the N terminus of TM8 extends significantly into the ER lumen, suggesting a high degree of flexibility in establishing TM8 transmembrane boundaries. These studies demonstrate that signal sequences can markedly influence stop transfer behavior and indicate that ER translocation machinery simultaneously integrates information from multiple topogenic determinants as they are presented in rapid succession during polytopic protein biogenesis.The topology of most eukaryotic polytopic proteins is generated in the endoplasmic reticulum (ER) 1 through the collective action of sequence determinants encoded within the nascent polypeptide. These determinants encompass hydrophobic transmembrane (TM) segments that, together with their flanking residues, interact with cytosolic and ER translocation machinery to initiate and terminate translocation and integrate the polypeptide into the lipid bilayer (reviewed in Refs. 1-3). In the simplest model, topology can be established cotranslationally by alternating topogenic determinants that function as signal (anchor) and stop transfer sequences (4 -7). As the first signal sequence emerges from the ribosome, it targets the ribosome nascent-chain complex (RNC) to the ER and gates open an aqueous channel in the membrane (the Sec61 translocon) (8). Because the ribosome exit site is directly aligned with the axial pore of the translocon, newly synthesized polypeptide is cotranslationally directed into the aqueous environment of the translocon as it emerges from the ribosome (9 -11). Subsequent synthesis of a stop transfer sequence gates the translocon closed to the ER lumen, terminates translocation, and provides the growing nascent polypeptide access to the cytosol (12, 13). Through sequential iterations of these events, signal and stop transfer sequences can alternately direct the polypeptide into the ER lumen or the cytosol and thus establish topology of transmembrane segments and lumenal and cytosolic peptide loops.Not all native polytopic proteins utilize a simple cotranslational biogenesis pathway. For ex...