Oligomerization state and supramolecular structure of the HIV-1 Vpu protein transmembrane segment in phospholipid bilayers

HIV-1 Vpu is an 81-residue protein with a single N-terminal transmembrane (TM) helical segment that is involved in the release of new virions from host cell membranes. Vpu and its TM segment form ion channels in phospholipid bilayers, presumably by oligomerization of TM helices into a pore-like structure. We describe measurements that provide new constraints on the oligomerization state and supramolecular structure of residues 1-40 of Vpu (Vpu(1-40)), including analytical ultracentrifugation measurements to investigate oligomerization in detergent micelles, photo-induced crosslinking experiments to investigate oligomerization in bilayers, and solid-state nuclear magnetic resonance measurements to obtain constraints on intermolecular contacts between and orientations of TM helices in bilayers. From these data, we develop molecular models for Vpu TM oligomers. The data indicate that a variety of oligomers coexist in phospholipid bilayers, so that a unique supramolecular structure can not be defined. Nonetheless, since oligomers of various sizes have similar intermolecular contacts and orientations, molecular models developed from our data are most likely representative of Vpu TM oligomers that exist in host cell membranes.