Assembly of Agrobacterium Phytochromes Agp1 and Agp2 with Doubly Locked Bilin Chromophores

The natural chromophore of most bacterial and fungal phytochromes is biliverdin (BV), which is incorporated in a covalent manner into the protein. Upon photoconversion between the red light-absorbing form Pr and the far-red light-absorbing form Pfr, the stereochemistry of the chromophore around the C 15 methine bridge changes from Z anti to E anti. Recombinant phytochromes Agp1 and Agp2 from Agrobacterium tumefaciens were assembled with a set of synthetic chromophores, including 2,18-Et-BV, 3,18-Et-BV, and the doubly locked 5Ea15Ea-BV, 5Es15Ea-BV, 5Za15Ea-BV, and 5Zs15Ea-BV. In all chromophores, covalent bond formation is restricted. As shown by spectral changes and desalting column separation, all chromophores are bound to Agp1 and Agp2. Adducts with 2,18-Et-BV and 3,18-Et-BV undergo normal photoconversion between Pr and Pfr. As opposed to typical phytochromes, the BV-Agp2 adduct converts from Pr to Pfr in darkness. However, the 2,18-Et-BV-Agp2 and 3,18-Et-BV-Agp2 adducts can undergo dark conversion from Pr to Pfr and Pfr to Pr, showing that ring A of the chromophore has a direct impact on the direction of dark conversion. The doubly locked chromophores were designed to probe for the stereochemistry of the C5 methine bridge in the Pfr form. The adducts with 5Es15Ea-BV and 5Zs15Ea-BV absorbed in the blue spectral range only. Therefore, the C5 E syn and Z syn stereochemistries are unlikely for the Pfr chromophore of Agp1 and Agp2. According to our spectra, the Agp2 chromophore most likely adopts an E anti stereochemistry at its C5 methine bridge. Thus, during Pr to Pfr conversion, the C5 methine bridge of the chromophore might undergo a Hula-twist isomerization. In Agp1, the Pfr chromophore is most likely in the C5 Z anti stereochemistry. We propose that the stereochemistry of the C5 methine bridge might differ between different phytochromes, most particularly in the Pfr form.