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. 2011 Mar 8;108(10):3958-63.
doi: 10.1073/pnas.1019668108. Epub 2011 Feb 14.

Transmembrane Orientation and Possible Role of the Fusogenic Peptide From Parainfluenza Virus 5 (PIV5) in Promoting Fusion

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Free PMC article

Transmembrane Orientation and Possible Role of the Fusogenic Peptide From Parainfluenza Virus 5 (PIV5) in Promoting Fusion

Jason E Donald et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

Membrane fusion is required for diverse biological functions ranging from viral infection to neurotransmitter release. Fusogenic proteins increase the intrinsically slow rate of fusion by coupling energetically downhill conformational changes of the protein to kinetically unfavorable fusion of the membrane-phospholipid bilayers. Class I viral fusogenic proteins have an N-terminal hydrophobic fusion peptide (FP) domain, important for interaction with the target membrane, plus a C-terminal transmembrane (C-term-TM) helical membrane anchor. The role of the water-soluble regions of fusogenic proteins has been extensively studied, but the contributions of the membrane-interacting FP and C-term-TM peptides are less well characterized. Typically, FPs are thought to bind to membranes at an angle that allows helix penetration but not traversal of the lipid bilayer. Here, we show that the FP from the paramyxovirus parainfluenza virus 5 fusogenic protein, F, forms an N-terminal TM helix, which self-associates into a hexameric bundle. This FP also interacts strongly with the C-term-TM helix. Thus, the fusogenic F protein resembles SNARE proteins involved in vesicle fusion by having water-soluble coiled coils that zipper during fusion and TM helices in both membranes. By analogy to mechanosensitive channels, the force associated with zippering of the water-soluble coiled-coil domain is expected to lead to tilting of the FP helices, promoting interaction with the C-term-TM helices. The energetically unfavorable dehydration of lipid headgroups of opposing bilayers is compensated by thermodynamically favorable interactions between the FP and C-term-TM helices as the coiled coils zipper into the membrane phase, leading to a pore lined by both lipid and protein.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Sequence conservation suggests a continuous helix including HRA and the FP. (A) Postfusion crystal structure of the soluble domain of closely related hPIV3 virus F protein (54). Shown in magenta is HRA. Below HRA, in the postfusion membrane, is the predicted location of the FP. (B) Heptad repeat of the FP and HRA. The beginning of the crystallographic resolved region of HRA is shown in magenta. Heptad repeats of small residues in the FP are boxed. (C) Sequence entropy of the FP and HRA can be fit to a single sinusoidal function with period of 3.47 ± 0.02 residues/turn (r = 0.51). (D) Sequence entropy of the FP alone can be fit to a single sinusoidal function with a period of 3.51 ± 0.08 residues/turn (r = 0.59).
Fig. 2.
Fig. 2.
ATR-IR of FP wild-type (A) and mutant Q120A (B) in phopholipid (POPC) bilayers. The sharp peak at 1656 cm-1 is indicative of alpha helical secondary structure. The TM orientation is demonstrated by the much greater intensity of the 1656 cm-1 amide I bond for parallel (0°) versus perpendicular (90°) polarized incident light (relative to the membrane normal).
Fig. 3.
Fig. 3.
Computational model of the PIV5 F FP hexameric bundle. (A) The Q120 residues form hydrogen bonds with one another as well as waters on the interior. (B) Side view shows the bundle oriented with the N-terminal end (which presumably faces the cellular interior) up. Water is shown in blue. Not shown for clarity are the phospholipids as well as one helix closest to the viewer.
Fig. 4.
Fig. 4.
Provisional model of PIV5 fusion. (A) Schematic diagrams of the limiting extremes of lipid-centric and pinprick fusion. (B) Shown is a model of the conformational change of the F protein (FP) hexamer (6HB) from a prehairpin, extended intermediate (Left) to a point of membrane apposition (Center) and finally to the postfusion state (Right). Proposed conformations of the FP 6HB are shown in the Insets along with 90° rotations. Note the increased tilt of the FP moving from the extended intermediate to the point of membrane apposition as well as the recruitment of lipid headgroups to the nascent pore. FPs are shown in red and blue, C-term-TMs are shown in magenta and yellow. The C-term-TM in the middle image contains two trimeric structures (57).

Comment in

  • A bundling of viral fusion mechanisms.
    Kasson PM, Pande VS. Kasson PM, et al. Proc Natl Acad Sci U S A. 2011 Mar 8;108(10):3827-8. doi: 10.1073/pnas.1101072108. Epub 2011 Feb 28. Proc Natl Acad Sci U S A. 2011. PMID: 21368165 Free PMC article. No abstract available.

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