Abstract: Transmembrane protein Slc11a1(Solute carrier family 11 member 1) is a divalent metal ion transporter which includes 12 transmembrane domains(TMDs). In this paper, the secondary structure and orientation of the peptides corresponding to TMD2, TMD3 and TMD4 in the phospholipid vesicles[n(DMPC)∶n(DMPG)=2∶1] at different pH values were investigated by circular dichroism, fluorescence, polarized attenuated total reflection Fourier transform infrared spectroscopies and differential scanning calorimetry. It was found that the secondary structure and positioning of TMD3 in lipid membrane are dependent on solution pH. TMD3 folds mainly as a β strand and is embedded in lipid membrane less deeply at pH=7, while it forms part of α-helix and inserts more deeply in lipid membrane at pH=5.5. These were attributed to the acidic residue Glu139 of TMD3, as indicated by the results of the E139A mutation of TMD3. Both TMD2 and TMD4 form predominantly α-helix structure at different pH values and insert into lipid membrane by tilt angles of 26° and 35°, respectively, and the positioning of them in membrane is basically independent of pH.

Key words: Solute carrier family 11, Secondary structure, Orientation, Spectroscopic measurement