The recently developed method of site-directed Fourier transform infrared dichroism for obtaining orientational constraints of oriented polymers is applied here to the transmembrane domain of the vpu protein from the human immunodeficiency virus type 1 (HIV-1). The infrared spectra of the 31-residue-long vpu peptide reconstituted in lipid vesicles reveal a predominantly α-helical structure. The infrared dichroism data of the 13C- labeled peptide yielded a helix tilt β = (6.5 ± 1.7)°from the membrane normal. The rotational pitch angle ω, defined as zero for a residue located in the direction of the helix tilt, is ω = (283 ± 11)°for the 13C labels Val13/Val20 and ω = (23 ± 11)°for the 13C labels Ala14/Val21. A global molecular dynamics search protocol restraining the helix tilt to the experimental value was performed for oligomers of four, five, and six subunits. From 288 structures for each oligomer, a left-handed pentameric coiled coil was obtained, which best fits the experimental data. The structure reveals a pore occluded by Trp residues at the intracellular end of the transmembrane domain.
Bibliographical noteFunding Information:
This work was supported by grants from the Welcome Trust and the BBSRC.