Structural studies of transmembrane domain4 of Nramp1

Abstract

Iron is an essential dietary trace metal required for a number of physiological and biochemical functions in the human body. It participates in a wide variety of metabolic processes such as oxygen transport. Since humans have no physiological pathway for iron excretion, the regulation of intestinal iron absorption is critical. The mammalian Nramp (natural resistance associated macrophage protein) gene family has been identified as the divalent metal ion transporter.1-3 In macrophage, Nramp1 (the analogue of DMT1/Nramp2) is recruited to the phagosomal membrane, where it may modulate divalent metal content to affect microbial replication. It is a membrane protein and has 12 putative transmembrane domains. It was suggested that transmembrane domain4 (TM4) play an important role in the transport function.2b In this work, the secondary structural features and membrane insertions of a peptide corresponding to the transmembrane domain4 (TM4) of Nramp1 in various membrane-mimicking environments were investigated. The peptide mainly adopts an alpha-helical structure in detergents such as SDS (sodium dodecyl sulphate) and DPC (dodecylphosphocholine) and lipids such as DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) and DMPG (1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)]). Our fluorescence study demonstrated that it shows more quenching effect in DMPC than in the presence of DMPG and different ratio of DMPC/DMPG. This shows that tryptophan in the fifth position of the peptide sequence is better protected in DMPG and mixed lipids.

We thank the Area of Excellence Scheme of University Grants Committee, University of Hong Kong (UGC) for their support. We are grateful to the University of Hong Kong for a Research Studentship (for M.K.).

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