Monomer-trimer equilibrium of the ectodomain of SIV gp41: Insight into themechanism of peptide inhibition of HIV infection

The monomer-trimer equilibrium of the ectodomain of SIV gp41 (residues 27-1 49, e-gp41) has been characterized by analytical ultracentrifugation, circu lar dichroism (CD), and NMR spectroscopy. Based on analytical ultracentrifu gation experiments performed at different rotor speeds and protein concentr ations, the equilibrium association constant for the STV e-gp41 trimer is 3 .1 X 10(11) M-2. The presence of intermolecular nuclear Overhauser effects in a mixture of C-12 and C-13-labeled e-gp41 prepared under nondenaturing c onditions unambiguously demonstrates that there is a dynamic equilibrium be tween the monomer and trimer. The CD spectra taken as a function of SIV e-g p41 concentration suggest that the helical content of the monomeric state d oes not change significantly relative to that of the trimeric state. The re levance of the monomer-trimer equilibrium is discussed with respect to gp41 function and the inhibitory properties of gp41 peptides.