The photochemistry of sodium ion pump rhodopsin observed by watermarked femto- to submillisecond stimulated Raman spectroscopy

Krokinobacter rhodopsin 2 (KR2) is a recently discovered light-driven Na+ pump that holds significant promise for application as a neural silencer in optogenetics. KR2 transports Na+ (in NaCl solution) or H+ (in larger cation solution, e.g. in CsCl) during its photocycle. Here, we investigate the photochemistry of KR2 with the recently developed watermarked, baseline-free femto- to submillisecond transient stimulated Raman spectroscopy (TSRS), which enables us to investigate retinal chromophore dynamics in real time with high spectral resolution over a large time range. We propose a new photocycle from femtoseconds to submilliseconds: J (formed in ∼200 fs) → K (∼3 ps) → K/L1 (∼20 ps) → K/L2 (∼30 ns) → L/M (∼20 μs). KR2 binds a Na+ ion that is not transported on the extracellular side, of which the function is unclear. We demonstrate with TSRS that for the D102N mutant in NaCl (with Na+ unbound, Na+ transport) and for WT KR2 in CsCl (with Na+ unbound, H+ transport), the extracellular Na+ binding significantly influences the intermediate K/L/M state equilibrium on the photocycle, while the identity of the transported ion, Na+ or H+, does not affect the photocycle. Our findings will contribute to further elucidation of the molecular mechanisms of KR2.