Solid-state NMR study of proteorhodopsin in the lipid environment: Secondary structure and dynamics

Proteorhodopsins are typical retinal-binding light-driven proton pumps of heptahelical architecture widely distributed in marine and freshwater bacteria. Recently, we have shown that green proteorhodopsin (GPR) can be prepared in a lipid-bound state that gives well-resolved magic angle spinning (MAS) NMR spectra in samples with different patterns of reverse labelling. Here, we present 3D and 4D sequential chemical shift assignments identified through experiments conducted on a uniformly 13C,15N-labelled sample. These experiments provided the assignments for 153 residues, with a particularly high density in the transmembrane regions (∼ 74% of residues). The extent of assignments permitted a detailed examination of the secondary structure and dynamics in GPR. In particular, we present experimental evidence of mobility of the protein's termini and of the A-B, C-D, and F-G loops, the latter being possibly coupled to the GPR ion-transporting function.