Research in the Rottner and Bieling laboratories is supported by the German Research Foundation (DFG) with grants to K.R. (GRK2223 and RO 2414/8-1) and P.B. (BI 1998/2-1), Helmholtz Association of German Research Centers (K.R.) and the Max Planck Society (P.B.). We thank M. Boiero Sanders (MPI Dortmund) for creating the molecular scheme in Fig. 1, based on published structures, and the members of our laboratories for input and discussions.