Temperature-scan cryocrystallography reveals reaction intermediates in bacteriophytochrome

Abstract

Phytochromes are photoreceptors that regulate light responses in plants, fungi and bacteria through reversible photoconversion between red and far-red light-absorbing states. Using temperature-scanning cryocrystallography, Yang et al. obtain structural information about three intermediates and key conformational changes that occur during the photoreaction of a bacteriophytochrome from Pseudomonas aeruginosa. Light is a fundamental signal that regulates important physiological processes such as development and circadian rhythm in living organisms. Phytochromes form a major family of photoreceptors responsible for red light perception in plants, fungi and bacteria1. They undergo reversible photoconversion between red-absorbing (Pr) and far-red-absorbing (Pfr) states, thereby ultimately converting a light signal into a distinct biological signal that mediates subsequent cellular responses2. Several structures of microbial phytochromes have been determined in their dark-adapted Pr or Pfr states3,4,5,6,7. However, the structural nature of initial photochemical events has not been characterized by crystallography. Here we report the crystal structures of three intermediates in the photoreaction of Pseudomonas aeruginosa bacteriophytochrome (PaBphP). We used cryotrapping crystallography to capture intermediates, and followed structural changes by scanning the temperature at which the photoreaction proceeded. Light-induced conformational changes in PaBphP originate in ring D of the biliverdin (BV) chromophore, and E-to-Z isomerization about the C15 = C16 double bond between rings C and D is the initial photochemical event. As the chromophore relaxes, the twist of the C15 methine bridge about its two dihedral angles is reversed. Structural changes extend further to rings B and A, and to the surrounding protein regions. These data indicate that absorption of a photon by the Pfr state of PaBphP converts a light signal into a structural signal via twisting and untwisting of the methine bridges in the linear tetrapyrrole within the confined protein cavity.