Evolutionary mix-and-match with MFS transporters II

Department of Physiology, Department of Microbiology, Immunology and Molecular Genetics, and Molecular Biology Institute, David Geffen School of Medicine, University of California, Los Angeles, CA 90095.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 11/2013; 110(50). DOI: 10.1073/pnas.1319754110
Source: PubMed


One fundamentally important problem for understanding the mechanism of coupling between substrate and H(+) translocation with secondary active transport proteins is the identification and physical localization of residues involved in substrate and H(+) binding. This information is exceptionally difficult to obtain with the Major Facilitator Superfamily (MFS) because of the broad sequence diversity of the members. The MFS is the largest and most diverse group of transporters, many of which are clinically important, and includes members from all kingdoms of life. A wide range of substrates is transported, in many instances against a concentration gradient by transduction of the energy stored in an H(+) electrochemical gradient using symport mechanisms, which are discussed herein. Crystallographic structures of MFS members indicate that a deep central hydrophilic cavity surrounded by 12 mostly irregular transmembrane helices represents a common structural feature. An inverted triple-helix structural symmetry motif within the N- and C-terminal six-helix bundles suggests that the proteins may have arisen by intragenic multiplication. In the work presented here, the triple-helix motifs are aligned in combinatorial fashion so as to detect functionally homologous positions with known atomic structures of MFS members. Substrate and H(+)-binding sites in symporters that transport substrates, ranging from simple ions like phosphate to more complex peptides or disaccharides, are found to be in similar locations. It also appears likely that there is a homologous ordered kinetic mechanism for the H(+)-coupled MFS symporters.

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    • "The result of this movement was to occlude a placed di-phenylalanine peptide in the binding site (Fig. 8B). The structural movement observed is very similar to that described above for the C-terminal helices TM10–11, suggesting that during peptide binding the transporter closes up around the peptide through the movement of individual helices, consistent with a recent analysis of other MFS structures [63] [64] and resembling an induced fit mechanism of binding. During the simulation, the movement reversed, suggesting that these two states are in equilibrium. "
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