The 2.0 Å Crystal Structure of a Pocilloporin at pH 3.5: The Structural Basis for the Linkage Between Color Transition and Halide Binding

• Published in: Photochemistry and photobiology Vol. 82; no. 2; pp. 359 - 366
• Main Authors: Wilmann, Pascal G., Battad, Jion, Beddoe, Travis, Olsen, Seth, Smith, Sean C., Dove, Sophie, Devenish, Rodney J., Rossjohn, Jamie, Prescott, Mark
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.03.2006
• Subjects: Symposium-in-Print
• ISSN: 0031-8655; 1751-1097
• DOI: 10.1562/2005-05-02-RA-509

The pocilloporin Rtms5 and an engineered variant Rtms5H146S undergo distinct color transitions (from blue to red to yellow to colorless) in a pH-dependent manner. pKa values of 4.1 and 3.2 were determined for the blue (absorption λmax, 590 nm) to yellow (absorption λmax, ∼453 nm) transitions of Rtms5 and Rtms5H146. The pKa for the blue-yellow transition of Rtms5H146S increased by 1.4 U in the presence of 0.1 M KI, whereas the pKa for the same transition of Rtms5 was relatively insensitive to added halides. To understand the structural basis for these observations, we have determined to 2.0 Å resolution the crystal structure of a yellow form of Rtms5H146S at pH 3.5 in the presence of iodide. Iodide was found occupying a pocket in the structure with a pH of 3.5, forming van der Waals contacts with the tyrosyl moiety of the chromophore. Elsewhere, it was determined that this pocket is occupied by a water molecule in the Rtms5H146S structure (pH 8.0) and by the side chain of histidine 146 in the wild-type Rtms5 structure. Collectively, our data provide an explanation for the observed linkage between color transitions for Rtms5H146S and binding to halides.