Dimerization properties of the RpBphP2 chromophore-binding domain crystallized by homologue-directed mutagenesis

Bacteriophytochromes (BphPs) are biliverdin IX alpha-containing photoreceptors that photoconvert between red (Pr) and far-red (Pfr) absorbing states. BphPs are one half of a two-component system that transmits a light signal to a histidine kinase domain and then to a gene-response regulator. In Rhodopseudomonas palustris, synthesis of a light-harvesting complex (LH4) is controlled by two BphPs (RpBphP2 and RpBphP3). Despite their high sequence identity (52%), their absorption spectra are very different. The spectra of RpBphP2 exhibit classic Pr-to-Pfr photoconversion, whereas RpBphP3 quenches and a high-energy Pnr state emerges [Giraud et al. (2005), J. Biol. Chem. 280, 32389-32397]. Crystallization of the chromophore-binding domain (CBD) of RpBphP2 (RpBphP2-CBD) proved to be difficult and the structure of RpBphP3-CBD was used to crystallize RpBphP2-CBD* using homologue-directed mutagenesis. The structure shows that dimerization is an important factor in successful crystallization of RpBphP2-CBD* and arises from an N136R mutation. Mutations at this site correlate with an ability to dimerize in other truncated BphPs and may also be important for full-length dimer formation. Comparison of the RpBphP3-CBD and RpBphP2-CBD* biliverdin IX alpha pockets revealed that the former has additional hydrogen bonding around the B and D pyrrole rings that may constrain photoconversion to Pfr, resulting in a strained photoexcited Pnr state.