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Folding of Aquaporin 1: Multiple evidence that helix 3 can shift out of the membrane core
Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics. Stockholm University, Science for Life Laboratory (SciLifeLab).
Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics. Stockholm University, Science for Life Laboratory (SciLifeLab).
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2014 (English)In: Protein Science, ISSN 0961-8368, E-ISSN 1469-896X, Vol. 23, no 7, 981-992 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2014. Vol. 23, no 7, 981-992 p.
Keyword [en]
membrane protein, translocon recognition, protein folding, hydrophobicity, molecular dynamics
National Category
Biochemistry and Molecular Biology
Identifiers
URN: urn:nbn:se:su:diva-106195DOI: 10.1002/pro.2483ISI: 000337669800014OAI: oai:DiVA.org:su-106195DiVA: diva2:735725
Note

AuthorCount:6;

Available from: 2014-07-31 Created: 2014-07-28 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Marginally hydrophobic transmembrane α-helices shaping membrane protein folding
Open this publication in new window or tab >>Marginally hydrophobic transmembrane α-helices shaping membrane protein folding
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Most membrane proteins are inserted into the membrane co-translationally utilizing the translocon, which allows a sufficiently long and hydrophobic stretch of amino acids to partition into the membrane. However, X-ray structures of membrane proteins have revealed that some transmembrane helices (TMHs) are surprisingly hydrophilic. These marginally hydrophobic transmembrane helices (mTMH) are not recognized as TMHs by the translocon in the absence of local sequence context.

We have studied three native mTMHs, which were previously shown to depend on a subsequent TMH for membrane insertion. Their recognition was not due to specific interactions. Instead, the presence of basic amino acids in their cytoplasmic loop allowed membrane insertion of one of them. In the other two, basic residues are not sufficient unless followed by another, hydrophobic TMH. Post-insertional repositioning are another way to bring hydrophilic residues into the membrane. We show how four long TMHs with hydrophilic residues seen in X-ray structures, are initially inserted as much shorter membrane-embedded segments. Tilting is thus induced after membrane-insertion, probably through tertiary packing interactions within the protein.

Aquaporin 1 illustrates how a mTMH can shape membrane protein folding and how repositioning can be important in post-insertional folding. It initially adopts a four-helical intermediate, where mTMH2 and TMH4 are not inserted into the membrane. Consequently, TMH3 is inserted in an inverted orientation. The final conformation with six TMHs is formed by TMH2 and 4 entering the membrane and TMH3 rotating 180°. Based on experimental and computational results, we propose a mechanism for the initial step in the folding of AQP1: A shift of TMH3 out from membrane core allows the preceding regions to enter the membrane, which provides flexibility for TMH3 to re-insert in its correct orientation.

Place, publisher, year, edition, pages
Stockholm: Department of biochemistry and biophysics, Stockholm University, 2014. 66 p.
Keyword
membrane protein folding, hydrophobicity, translocon, transmembrane helix, marginally hydrophobic transmembrane helices, orientational preference, positive inside rule, aquaporin 1
National Category
Biochemistry and Molecular Biology
Research subject
Biochemistry
Identifiers
urn:nbn:se:su:diva-109335 (URN)978-91-7649-050-1 (ISBN)
Public defence
2014-12-19, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrheniusväg 16 B, Stockholm, 13:00 (English)
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Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Available from: 2014-11-27 Created: 2014-11-18 Last updated: 2014-11-28Bibliographically approved

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