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

Abstract

The pocilloporin Rtms5 and an engineered variant Rtms5 H146S 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 Rtms5 H146. The pKa for the blue-yellow transition of Rtms5 H146S 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 Rtms5 H146S 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 Rtms5 H146S 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 Rtms5 H146S and binding to halides.