1 Department of Biochemistry, University of Oxford, South Parks Rd, Oxford OX13QC, UK. Electronic address: simon.newstead@bioch.ox.ac.uk.
2 Institute of Biochemistry, Biocenter, Goethe University Frankfurt, Max-von-Laue Str. 9, Frankfurt/Main 60438, Germany. Electronic address: tampe@em.uni-frankfurt.de.