Folding of Aquaporin 1: multiple evidence that helix 3 can shift out of the membrane core
: Virkki MT, Agrawal N, Edsbäcker E, Cristobal S, Elofsson A, Kauko A
: 2014
: Protein Science
: Protein science : a publication of the Protein Society
: Protein Sci
: 23
: 7
: 981
: 92
: 0961-8368
: 1469-896X
DOI: https://doi.org/10.1002/pro.2483(external)
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.