The conformational changes during the photocycle of
the photoactive yellow protein have been the subject
of many recent studies. Spectroscopic measurements have
shown that the photocycle also occurs in a crystalline
environment, and this has been the basis for subsequent
Laue diffraction and cryocrystallographic studies. These
studies have shown that conformational changes during the
photocycle are limited to the chromophore and its immediate
environment. However, spectroscopic studies suggest the
presence of large conformational changes in the protein.
Here, we address this apparent discrepancy in two ways.
First, we obtain a description of large concerted motions
in the ground state of the yellow protein from NMR data
and theoretical calculations. Second, we describe the high-resolution
structure of the yellow protein 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.