1. Department of Biochemistry of Membranes, Centre for Biomembranes and Lipid Enzymology, Institute of Biomembranes, Padualaan 8, 3584 CH Utrecht, The Netherlands, and Department of Biomolecular Mass Spectrometry, Bijvoet Centre for Biomolecular Research and Utrecht Institute for Pharmaceutical Sciences, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands