The atomic coordinates and structure factors (codes 3N3N, 3N3O, 3N3P, 3N3Q, 3N3R, and 3N3S) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/). * This work was supported by a...