Isotropic bicelles stabilize the juxtamembrane region of the influenza M2 protein for solution NMR studies

The protein M2 from influenza is a tetrameric membrane protein with several roles in the viral life cycle. The transmembrane helix (TMH) of M2 has proton channel activity that is required for unpackaging the viral genome. Additionally a C-terminal juxtamembrane region includes an amphipathic helix (APH) important for virus budding and scission. The APH interacts with membranes and is required for M2 localization to the site of viral budding. As a step toward obtaining high resolution information on the structure and lipid interactions of the M2 APH, we sought to develop a fast tumbling bicelle system, which would make studies of M2 in a membrane-like environment by solution NMR possible. Since M2 is highly sensitive to the solubilizing environment, an M2 construct containing the APH was studied under micelle and bicelle conditions while maintaining the same detergent and lipid headgroup chemistry to facilitate interpretation of the spectroscopic results. The sequence from a human H1N1 "swine flu" isolate was used to design an M2 construct (swM2) similar in amino acid sequence to currently circulating viruses. Comparison of swM2 solubilized in either the diacyl detergent 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) or a mixture of DHPC and the lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) (q = 0.4) indicated that the largest changes were a decrease in helicity at the N-terminus of the TMH and a decrease in dynamics for the juxtamembrane linker residues connecting the TMH and the APH. Whereas the linker region is very dynamic and the amide protons are rapidly exchanged with water protons in micelles, the dynamics and water exchange are largely suppressed in the presence of lipid. Chemical shift changes and relaxation measurements were consistent with an overall stabilization of the linker region, with only modest changes in conformation or environment of the APH itself. Such changes are consistent with differences observed in structures of M2 in lipid bilayers and detergent micelles, indicating that the bicelle system provides a more membrane-like environment.