1. Signaling Systems Laboratory, Department of Chemistry and Biochemistry and Bioinformatics Graduate Program, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093–0375, USA
2. The Whitaker Institute for Biomedical Engineering, Department of Biomedical Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
3. Signaling Systems Laboratory, Department of Chemistry and Biochemistry, University of California, San Diego
4. The Whitaker Institute for Biomedical Engineering, Department of Biomedical Engineering, Johns Hopkins University