Controlled reduction of the humidity induces a shortcut recovery reaction in the photocycle of photoactive yellow protein

The photocycle of the blue-light photoreceptor protein Photoactive Yellow Protein (PYP) was studied at reduced relative humidity (RH). Photocycle kinetics and spectra were measured in thin films of PYP in which the relative humidity was set at values between 29 and 98% RH with saturated solutions of various salts. We show that in this range, approximately 200 water molecules per PYP molecule are released from the film. As humidity decreased, photocycle transition rates changed, until at low humidity (RH < 50%) an authentic photocycle was no longer observed and the absorption spectrum of the dark, equilibrium state of PYP started to shift to 355 nm, that is, to a form resembling that of pB(dark). At moderately reduced humidity (i.e., > 50% RH), an authentic photocycle is still observed, although its characteristics differ from those in solution. As humidity decreases, the rate of ground state recovery increases, while the rate of depletion of the first red-shifted intermediate pR dramatically decreases. The latter observation contrasts all so-far known modulations of the rate of the transition of the red-shifted- to the blue-shifted intermediates of PYP, which is consistently accelerated by all other modulations of the mesoscopic context of the protein. Under these same conditions, the long-lived, blue-shifted intermediate was formed not only with slower kinetics than in solution but also to a smaller extent. Global analysis of these data indicates that in this low humidity environment the photocycle can take a different route than in solution, that is, part of pG recovers directly from pR. These experiments on wild-type PYP, in combination with observations on a variant of PYP obtained by site-directed mutagenesis (the E46Q mutant protein), further document the context dependence of the photocycle transitions of PYP and are relevant for the interpretation of results obtained in both spectroscopic and diffraction studies with crystalline PYP.