Krokinobacter eikastus rhodopsin 2 (KR2) is a pentameric, light-driven ion pump, which selectively transports sodium or protons. The mechanism of ion selectivity and transfer is unknown. By using conventional as well as dynamic nuclear polarization (DNP)-enhanced solid-state NMR, we were able to analyse the retinal polyene chain between positions C10 and C15 as well as the Schiff base nitrogen in the KR2 resting state. In addition, 50% of the KR2 13C and 15N resonances could be assigned by multidimensional high-field solid-state NMR experiments. Assigned residues include part of the NDQ motif as well as sodium binding sites. Based on these data, the structural effects of the H30A mutation, which seems to shift the ion selectivity of KR2 primarily to Na+, could be analysed. Our data show that it causes long-range effects within the retinal binding pocket and at the extracellular Na+ binding site, which can be explained by perturbations of interactions across the protomer interfaces within the KR2 complex. This study is complemented by data from time-resolved optical spectroscopy.
Keywords: DNP; H30A; KR2; Membrane proteins; Microbial rhodopsin; Optical spectroscopy; Photocycle; Resonance assignment; Sodium pump; Solid-state NMR.
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