Thinking outside the crystal: complementary approaches for examining transporter conformational change

As the number of high-resolution structures of membrane proteins continues to rise, so has the necessity for techniques to link this structural information to protein function. In the case of transporters, function is achieved via coupling of conformational changes to substrate binding and release. Static structural data alone cannot convey information on these protein movements, but it can provide a high-resolution foundation on which to interpret lower resolution data obtained by complementary approaches. Here, we review selected biochemical and spectroscopic methods for assessing transporter conformational change. In addition to more traditional techniques, we present ¹⁹F-NMR as an attractive method for characterizing conformational change in transporters of known structure. Using biosynthetic labeling, multiple, non-perturbing fluorine-labeled amino acids can be incorporated throughout a protein to serve as reporters of conformational change. Such flexibility in labeling allows characterization of movement in protein regions that may not be accessible via other methods.