1. Department of Biochemistry, Institute for Biology, University of Kassel, Kassel 34132, Germany
2. Department of Pharmacology, University of California, San Diego, 9400 Gilman Drive, La Jolla, California 92093-0654, United States
3. Department of Chemistry and Biochemistry, University of California, San Diego, 9400 Gilman Drive, La Jolla, California 92093-0654, United States
4. The Norwegian Structural Biology Centre, Department of Chemistry, UiT the Arctic University of Norway, Tromsø 9012, Norway