The complex of sensory rhodopsin II (NpSRII) with its cognate transducer (NpHtrII) mediates negative
phototaxis in halobacteria Natronomonas pharaonis. Upon light activation NpSRII triggers, by means of
NpHtrII, a signal transduction chain homologous to the two component system in eubacterial chemotaxis.
Here we report on the crystal structure of the ground state of the mutant NpSRII-D75N/NpHtrII
complex in the space group I212121. Mutations of this aspartic acid in light-driven proton pumps dramatically
modify or/and inhibit protein functions. However, in vivo studies show that the similar D75N mutation
retains functionality of the NpSRII/NpHtrII complex. The structure provides the molecular basis for
the explanation of the unexpected observation that the wild and the mutant complexes display identical
physiological response on light excitation.