J. Bacteriol., Aug 1995, 4579-4586, Vol 177, No. 16
K Okamoto, T Baba, H Yamanaka, N Akashi and Y Fujii
The Escherichia coli heat-stable enterotoxin II (STII) is a typical
extracellular toxin consisting of 48 amino acid residues, of which 4 are
cysteine. There are two disulfide bonds, one between Cys-10 and Cys- 48 and
one between Cys-21 and Cys-36. We examined the involvement of DsbA in the
formation of the disulfide bonds of STII and the role of each in the
secretion of STII. A dsbA mutant was transformed with a plasmid harboring
the STII gene, and STII was not detected either in the cells or in the
culture supernatant. Reducing the level of STII brought about the dsbA
mutation restored by introducing the wild-type dsbA gene into the mutant
strain. These results showed that DsbA is involved in forming the disulfide
bonds of STII and that STII without these disulfide bonds is degraded
during secretion. We substituted these four cysteine residues in vivo by
oligonucleotide-directed site- specific mutagenesis. The amino acid
sequence of the purified STII (C48S) and pulse-chase studies revealed that
two intermolecular disulfide bonds must be formed to be efficiently
secreted and that cleavage between amino acid residues 14 and 15 is
probably the first step in the proteolytic degradation of STII.
Copyright © 1995, American Society for Microbiology
Disulfide bond formation and secretion of Escherichia coli heat-stable enterotoxin II
Department of Biochemistry, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Japan.
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