Conformational substates in different crystal forms of the photoactive yellow protein - Correlation with theoretical and experimental flexibility

The conformational changes during the photocycle of the photoactive yellow protein have been the subject of many recent studies. Spectroscopic measure ments have shown that the photocycle also occurs in a crystalline environme nt, and this has been the basis for subsequent Laue diffraction and cryocry stallographic studies. These studies have shown that conformational changes during the photocycle are limited to the chromophore and its immediate env ironment. However, spectroscopic studies suggest the presence of large conf ormational changes in the protein. Here, we address this apparent discrepan cy in two ways. First, we obtain a description of large concerted motions i n the ground state of the yellow protein from NMR data and theoretical calc ulations. Second, we describe the high-resolution structure of the yellow p rotein crystallized in a different space group. The structure of the yellow protein differs significantly between the two crystal forms. We show that these differences can be used to obtain a description of the flexibility of the protein that is consistent with the motions observed in solution.