ACCESSION NUMBERS Coordinates and the structure factors have been deposited in the Protein Data Bank with accession codes 3VAF (BrU3), 3VAG (BrU3C2), 3VAH [(BrU3)C4], 3VAI [(BrU3)C5], 3VAK [BrU5], 3VA... [(BrU5)C1], 3VAM [(BrU5)C2], 3VAJ [(BrU5)C6] (on hold for publication).