Transverse and tangential orientation of predicted transmembrane fragments 4 and 10 from the human multidrug resistance protein (hMRP1/ABCC1) in membrane mimics

The human multidrug-resistance-associated protein 1 (hMRP1/ABCC1) belongs to the large ATP-binding cassette transporter superfamily. In normal tissues, hMRP1 is involved in tissue defense, whereas, in cancer cells, it is overproduced and contributes to resistance to chemotherapy. We previously investigated the folding properties of the predicted transmembrane fragments (TM) TM16, and TM17 from membrane-spanning domain 2 (MSD2). These TMs folded only partially as an alpha-helix and were located in the polar headgroup region of detergent micelles used as membrane mimics (Vincent et al. in Biochim Biophys Acta 1768:538-552, 2007; de Foresta et al. in Biochim Biophys Acta 1798:401-414, 2010). We have now extended these studies to TM4 and TM10, from MSD0 and MSD1, respectively. TM10 may be involved in the substrate translocation pathway whereas the role of TM4 is less predictable, because few studies have focused on MSD0, a domain present in some hMRP1 homologs only. Each TM contained a single Trp residue (W142 or W553) acting as an intrinsic fluorescent probe. The location and dynamics of the TMs in dodecylphosphocholine (DPC) or n-dodecyl-beta-d-maltoside (DDM) micelles were studied by Trp steady-state and time-resolved fluorescence, including quenching experiments. Overall TM structure was analyzed by far-UV circular dichroism studies in detergent micelles and TFE. TM10 behaved similarly to TM16 and TM17, with an interfacial location in micelles consistent with a possible role in lining the transport pore. By contrast, TM4 behaved like a classical TM fragment with a high alpha-helical content, and its transmembrane insertion did not require its interaction with other TMs.