1. Signaling Systems Laboratory, Department of Microbiology, Immunology, and Molecular Genetics, UCLA, Los Angeles, CA 90095, USA.
2. Institute for Quantitative and Computational Biosciences, UCLA, Los Angeles, CA 90095, USA.
3. Brighton and Sussex Medical School, Department of Clinical and Experimental Medicine, University of Sussex, Falmer, East Sussex BN1 9PX, UK.