Folding of Aquaporin 1: multiple evidence that helix 3 can shift out of the membrane core.
The folding of most integral membrane proteins follows a two-step process: initially, individual transmembrane helices are inserted into the membrane by the Sec translocon. Thereafter, these helices fold to shape the final conformation of the protein. However, for some proteins, including Aquaporin 1 (AQP1), the folding appears to follow a more complicated path. AQP1 has been reported to first insert as a four-helical intermediate, where helix 2 and 4 are not inserted into the membrane. In a second step, this intermediate is folded into a six-helical topology. During this process, the orientation of the third helix is inverted. Here, we propose a mechanism for how this reorientation could be initiated: first, helix 3 slides out from the membrane core resulting in that the preceding loop enters the membrane. The final conformation could then be formed as helix 2, 3, and 4 are inserted into the membrane and the reentrant regions come together. We find support for the first step in this process by showing that the loop preceding helix 3 can insert into the membrane. Further, hydrophobicity curves, experimentally measured insertion efficiencies and MD-simulations suggest that the barrier between these two hydrophobic regions is relatively low, supporting the idea that helix 3 can slide out of the membrane core, initiating the rearrangement process.
Department of Biochemistry and Biophysics and Science for Life Laboratory, Stockholm University, Solna, SE-171 21, Sweden.
Minttu T
MT
Virkki
Nitin
N
Agrawal
Elin
E
Edsbäcker
Susana
S
Cristobal
Arne
A
Elofsson
Anni
A
Kauko
D000818:Animals; D051398:Aquaporin 1 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D051401:Aquaporin 4 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D002462:Cell Membrane; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D056004:Molecular Dynamics Simulation; D017354:Point Mutation; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D051381:Rats
D051401:Aquaporin 4; D051398:Aquaporin 1
hydrophobicity; membrane protein; molecular dynamics; protein folding; translocon recognition