Tryptophan fluorescence as a reporter for structural changes in photoactive yellow protein elicited by photo-activation

Light-activation of photoactive yellow protein (PYP) is followed by a series of dynamical transitions in the structure of the protein. Tryptophan fluorescence is well-suited as a tool to study selected aspects of these. Using site-directed mutagenesis eight 'single-tryptophan' mutants of PYP were made by replacement of either a tyrosine, phenylalanine or histidine residue by tryptophan, while simultaneously eliminating the endogenous W119. Surprisingly, only three of these eight mutants turn out to emit measurable tryptophan fluorescence: F6W/W119F, F96W/W119F and H108W/W119F. Significantly, all three show altered tryptophan fluorescence upon formation of the pB state. As F96 is located very close to the chromophore, the F96W/W119F mutant protein is particularly suitable for further studies on the dynamical changes of the polarity in the chromophore-binding pocket of PYP. Furthermore, WT PYP can be photo-activated by a UV photon via the highly conserved W119 and subsequent Forster resonance energy transfer. Placing a unique tryptophan residue elsewhere in the protein shows that position 119 is favoured for UV-activation of PYP.