Picometer-scale conformational heterogeneity separates functional from non-functional states of a photoreceptor protein

We combined ultra-high resolution X-ray crystallography, cryogenic trapping and in-crystal photoactivation to show that subtle picometer-scale variations in protein structure have dramatic effects on the function of the bacterial photoreceptor PYP. Specifically we show that what appears to be a homogeneous structural population – even in a 0.82A crystal structure – is in fact a mix of highly active and inactive molecular states. Of particular note is that the light-sensing activity of the overall photoreceptor population is wholly attributable to a minor conformational substate, while the dominant structural state is inactive. Interestingly, the molecules that show photo activity are those, in which the displacement of active-site atoms anticipate the trajectory of the photoactivation reaction. Further, our results indicate that the molecule-to-molecule functional heterogeneity observed in a series of recent single-molecule experiments may be caused by structural differences that are too sublte to be obse...