Structure, Assembly, and Topology of the G185R Mutant of the Fourth Transmembrane Domain of Divalent Metal Transporter

The mammalian iron transporter, divalent metal transporter (DMT1), is a 12-transmembrane domain integral protein, responsible for dietary iron uptake in the duodenum and iron acquisition from transferrin in peripheral tissues. Two disease-causing mutants in animals have been found and attributed to the same missense mutation (G185R), which occurs within the putative transmembrane domain 4 (TM4) of DMT1. We have characterized a synthetic 24-mer peptide, corresponding to the sequence of the TM4 of DMT1 with G185R mutation using circular dichroism (CD) and NMR spectroscopy and show that the G185R peptide assumes mainly α-helical conformations in various membrane-mimetic environments. Solution structures derived from NMR and molecular dynamics/simulated annealing calculations demonstrate that the peptide exhibits a highly defined α-helix in its middle portion, flanked by a highly flexible N-terminus and a relatively ordered C-terminus. Both the folding and location of the C-terminus in SDS micelles are regulated by pH values. Paramagnetic broadening on peptide NMR signals by spin-labeled 5- and 16-doxylstearic acids and Mn²⁺ ion suggests that both the N-terminus and the helical region of the peptide are embedded in SDS micelles. Surprisingly, self-association of the peptides for both the wild type and the G185R mutant studied by CD, electrospray ionization mass spectrometry, and NMR diffusion-ordered spectroscopy demonstrated that mutation of the Gly185 to a bulky and positively charged arginine causes a different self-assembly of the peptide, e.g., from a trimer to a hexamer, which implies that the quaternary structure of integral DMT1 may be crucial for its function in vivo.