The heat-labile enterotoxin from Escherichia coli (LT) is a member of the cholera toxin family. These and other members of the larger class of AB5 bacterial toxins act through catalyzing the ADP-ribosylation of various intracellular targets including Gs alpha ...
The heat-labile enterotoxin from Escherichia coli (LT) is a member of the cholera toxin family. These and other members of the larger class of AB5 bacterial toxins act through catalyzing the ADP-ribosylation of various intracellular targets including Gs alpha. The A subunit is responsible for this covalent modification, while the B pentamer is involved in receptor recognition. We report here the crystal structure of an inactive single-site mutant of LT in which arginine 7 of the A subunit has been replaced by a lysine residue. The final model contains 103 residues for each of the five B subunits, 175 residues for the A1 subunit, and 41 residues for the A2 subunit. In this Arg7Lys structure the active site cleft within the A subunit is wider by approximately 1 A than is seen in the wild-type LT. Furthermore, a loop near the active site consisting of residues 47-56 is disordered in the Arg7Lys structure, even though the new lysine residue at position 7 assumes a position which virtually coincides with that of Arg7 in the wild-type structure. The displacement of residues 47-56 as seen in the mutant structure is proposed to be necessary for allowing NAD access to the active site of the wild-type LT. On the basis of the differences observed between the wild-type and Arg7Lys structures, we propose a model for a coordinated sequence of conformational changes required for full activation of LT upon reduction of disulfide bridge 187-199 and cleavage of the peptide loop between the two cysteines in the A subunit.(ABSTRACT TRUNCATED AT 250 WORDS)
Related Citations:
Probing the Structure-Activity Relationship of Escherichia Coli Lt-A by Site-Directed Mutagenesis Pizza, M., Domenighini, M., Hol, W., Giannelli, V., Fontana, M.R., Giuliani, M.M., Magagnoli, C., Peppoloni, S., Manetti, R., Rappuoli, R. (1994) Mol Microbiol 14: 51
Refined Structure of Escherichia Coli Heat-Labile Enterotoxin, a Close Relative of Cholera Toxin Sixma, T.K., Kalk, K.H., Van Zanten, B.A.M., Dauter, Z., Kingma, J., Witholt, B., Hol, W.G.J. (1993) J Mol Biol 230: 890
Crystal Structure of a Cholera Toxin-Related Heat-Labile Enterotoxin from E. Coli Sixma, T.K., Pronk, S.E., Kalk, K.H., Wartna, E.S., Van Zanten, B.A.M., Witholt, B., Hol, W.G.J. (1991) Nature 351: 371
Organizational Affiliation:
Department of Biological Structure and Biochemistry, University of Washington, Seattle, USA.
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