Po. Falnes et S. Olsnes, CELL-MEDIATED REDUCTION AND INCOMPLETE MEMBRANE TRANSLOCATION OF DIPHTHERIA-TOXIN MUTANTS WITH INTERNAL DISULFIDES IN THE A-FRAGMENT, The Journal of biological chemistry, 270(35), 1995, pp. 20787-20793
Active diphtheria toxin consists of two fragments, A and B, joined by
a disulfide bond. The B fragment binds to cell surface receptors and a
ids in the translocation of the enzymatically active A fragment to the
cytosol. Normally, the toxin A fragment enters the cytosol from acidi
c endosomes, but translocation can also be induced at the level of the
plasma membrane by exposing cells with surface-bound toxin to low pH.
Recently, we showed that disulfide bonds introduced into the A fragme
nt by mutation are inhibitory for translocation. In the present work,
we found that although the complete translocation of the A fragment is
blocked, three mutant toxins underwent reduction of the interfragment
disulfide bond upon low pH exposure, whereas the internal disulfide i
n the A fragment remained intact. In the case of two of these mutants,
the A fragment was released into the extracellular medium upon exposu
re of cell-bound toxin to low pH. The pH profile for the release of th
e mutant A fragments was the same as for translocation of wild-type A
fragment to the cytosol, and the release was inhibited by conditions t
hat interfere with A fragment translocation. In the case of the third
mutant, which remained cell-associated upon reduction of the interfrag
ment disulfide bond, a translocation intermediate was detected. The re
sults show that the reduction of the interfragment disulfide bond can
occur in the absence of complete translocation of the A fragment to th
e cytosol, and they indicate that the reduction takes place at an earl
y stage in the translocation process. Our findings suggest that the tr
anslocation of the A fragment across the membrane is initiated at the
C terminus.