Folding and signaling share the same pathway in a photoreceptor

The photoreceptor photoactive yellow protein (PYP) was used as a model syst em to study receptor activation and protein folding. Refolding was studied by stopped-flow absorbance spectroscopy for PYP with either a traps or a ci s chromophore. Chromophore traps to cis isomerization, the mechanism of lig ht detection by PYP, greatly affects the protein folding process. When the cis chromophore is present, refolding from the unfolded state proceeds thro ugh the putative signaling state of PYP as an on-pathway intermediate. In a ddition, moderate denaturant concentrations result in the specific unfoldin g of the signaling state of PYP. Thus, the signaling state is common to the pathways of folding and signaling. This result provides an avenue for the study of protein folding. We demonstrate how this approach can be used to e stablish whether a folding intermediate is on-pathway or off-pathway. The r esults also reveal transient partial unfolding as a molecular mechanism for signaling.