Figure 1.

Two possible dissociation pathways ([1] and [2]) of a hypothetical dimeric protein consisting of four transmembrane helices per subunit (blue) and a cytoplasmic subunit (red) in a detergent micelle (green). If the detergent concentration is depleted, activation removes detergent and disrupts subunit interactions - pathway [1] is followed. Mass spectra are shown for the membrane protein EmrE. When detergent was depleted from solution containing EmrE, prior to analysis, a broad heterogeneous distribution of detergent aggregates is formed (i). After tandem MS of a defined m/z range (blue line (i)) detergent ions, ligand (TPP⁺) monomeric EmrE are observed (ii). If the detergent concentration is maintained above the CMC, and activated once in the gas phase (pathway [2]), not only is the quality of the spectra much improved (iii) but subunit interactions can also be maintained (iv). This spectrum of dimeric EmrE (iv) reveals lipid binding and PTMS (N-formyl methionine). Blue lines represent the statistical incorporation of modified and wild type subunits and are compared with data in the +7 and +9 charge states. Binding of one or two lipids is clearly observed for the protein dimer, but not for the monomer, implying that the conditions employed to preserve subunit interactions also preserve lipid binding.