The Structure of the Mercury Transporter MerF in Phospholipid Bilayers: A Large Conformational Rearrangement Results from N-Terminal Truncation

The three-dimensional structure of the 81-residue mercury transporter MerF determined in liquid crystalline phospholipid bilayers under physiological conditions by Rotationally Aligned (RA) solid-state NMR has two long helices, which extend well beyond the bilayer, with a well-defined interhelical loop. Truncation of the N-terminal 12 residues, which are mobile and unstructured when the protein is solubilized in micelles, results in a large structural rearrangement of the protein in bilayers. In the full-length protein, the N-terminal helix is aligned nearly parallel to the membrane normal and forms an extension of the first transmernbrane helix. By contrast, this helix adopts a perpendicular orientation in the truncated protein. The close spatial proximity of the two Cys-containing metal binding sites in the three-dimensional structure of full-length MerF provides insights into possible transport mechanisms These results demonstrate that major changes in protein structure can result from differences in amino acid sequence (e.g., full-length vs truncated proteins) as well as the use of a non-native membrane mimetic environment (e.g., micelles) vs liquid crystalline phospholipid bilayers. They provide further evidence of the importance of studying unmodified membrane proteins in near native bilayer environments in order to obtain accurate structures that can be related to their functions.