Heat-labile enterotoxin in Escherichia coli. Kinetics of association of subunits into periplasmic holotoxin.
We have investigated the assembly of the heat-labile enterotoxin (LT) subunits after their processing and segregation into the periplasmic space as mature LT A and LT B polypeptides. LT B starts associating into oligomers during or immediately after translocation through the cytoplasmic membrane. Binding to LT A occurs immediately after oligomerization. Over 80% of the LT B subunits have oligomerized, and over 50% have associated with LT A into holotoxin within 1 min after synthesis. The fate of newly synthesized LT A is totally different. There is an extensive overproduction of LT A relative to LT B and after membrane translocation it becomes part of a periplasmic pool of free LT A. It is then bound by LT B oligomers or degraded at such a rate that the free periplasmic LT A disappears from the pool with a half-time of 20-25 min. About half of the LT A is incorporated into holotoxin, while the other half is degraded. We conclude that LT subunits are translocated and processed in a ratio of about 2 A to 5 B. Since free LT A is either degraded slowly or bound to newly synthesized LT B oligomers, the net result is a steady state of 1.4 to 1.7 A subunits to 5 B subunits in the periplasm. About 60% of this LT A is bound by LT B to form periplasmic holotoxin with a subunit ratio of about 1 A to 5 B. The remaining 40% of periplasmic LT A occurs free.
D001427:Bacterial Toxins / Q000032:analysis*; D004591:Electrophoresis, Polyacrylamide Gel; D004768:Enterotoxins / Q000032:analysis*; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins; D007700:Kinetics; D046911:Macromolecular Substances; D008715:Methionine / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism
D001427:Bacterial Toxins; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D046911:Macromolecular Substances; D014443:Tyrosine; D008715:Methionine; C027538:heat-labile enterotoxin, E coli