Abstract
Cryo-electron microscopy and X-ray crystallography have shown that the pre- and postfusion states of the HIV-1 gp41 viral coat protein, although very different from one another, each adopt C(3) symmetric structures. A stable homotrimeric structure for the transmembrane domain (TM) also was modeled and supported by experimental data. For a C(3) symmetric structure, alignment in an anisotropic medium must be axially symmetric, with the unique axis of the alignment tensor coinciding with the C(3) axis. However, NMR residual dipolar couplings (RDCs) measured under three different alignment conditions were found to be incompatible with C(3) symmetry. Subsequent measurements by paramagnetic relaxation enhancement, analytical ultracentrifugation, and DEER EPR, indicate that the transmembrane domain is monomeric. (15)N NMR relaxation data and RDCs show that TM is highly ordered and uninterrupted for a total length of 32 residues, extending well into the membrane proximal external region.