title: pH-induced transitions in cholera toxin conformation: a fluorescence study.
abstract: Determination of the ratio of intrinsic fluorescence with dibrominated Bry 96 (F) relative to that with unbrominated Bry 96 (F0), at neutral pH and in the presence of 0.2 M NaCl, reveals that the A subunit of cholera toxin (CT A) has a somewhat higher affinity for this mild detergent than intact cholera toxin (CT) and its B subunit (CT B). Receptor (GM1 or oligo-GM1) binding has no influence on the very low detergent binding of CT and CT B. Activation of CT A by treatment with dithiothreitol (20 mM) also does not affect detergent binding. The weak hydrophobic nature of CT A is also reflected by the negative modulatory action of anionic phospholipids and deoxycholate on its mono-ADP-ribosyltransferase activity and the ability of the former to decrease its intrinsic fluorescence intensity in a salt-resistant way. Detergent binding of CT A is only slightly pH dependent whereas, upon lowering the pH, detergent binding to CT or CT B becomes significant. In the pH range 6.5-4.2 a gradual increase in detergent binding to CT and CT B occurs. In the narrow pH range 4.2-4.0 a sharp and time-dependent enhancement of brominated Bry 96 quenching is observed. The increase in detergent binding upon lowering the pH is fully reversible, salt dependent, and complete within 10 min (t1/2 = 2 min at 25 degrees C). Solute quenching experiments with the neutral polar quencher acrylamide reveal that upon lowering the pH to 5.0 a marked increase in the exposure of the lone Trp-88 residue in each beta-polypeptide chain of CT B occurs.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D020106:Acrylamide; D000178:Acrylamides / Q000494:pharmacology; D002772:Cholera Toxin / Q000378:metabolism*; D003902:Detergents / Q000494:pharmacology; D005453:Fluorescence; D006863:Hydrogen-Ion Concentration; D010743:Phospholipids / Q000494:pharmacology; D011485:Protein Binding; D011487:Protein Conformation / Q000187:drug effects; D012492:Salts / Q000494:pharmacology; D012965:Sodium Chloride / Q000494:pharmacology
chemical_list: D000178:Acrylamides; D003902:Detergents; D010743:Phospholipids; D012492:Salts; D020106:Acrylamide; D012965:Sodium Chloride; D002772:Cholera Toxin