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Some of the amino acid residues in Figs. 2 and 3 of this paper were incorrectly labeled. In Fig. 2a, residue 535 should be a glutamate (E535); in Fig. 2d, residue 191 should be a threonine (T191) and residue 192 should be a tyrosine (Y192); and in Fig. 3a, residue 798 should be a phenyl-alanine (Phe798). The panels with corrected labels are printed here. The authors apologize for any inconvenience this may have caused.