Intracellular trafficking and membrane translocation of pertussis toxin into host cells

The translocation of the pertussis toxin (PTX) S1 subunit into the cytoplas m of host cells was analysed in CHO cells producing S1 fused to a signal pe ptide. This protein channelled into the endoplasmic reticulum (ER) by the s ignal peptide, was found to ADP-ribosylate its target G proteins, suggestin g that membrane translocation can occur from the ER and does not require th e B oligomer. Similar results were obtained with a C-terminally truncated S 1 subunit, indicating that this hydrophobic tail is not involved in the tra nslocation mechanism. We also analysed the activity of two PTX mutants in w hich the S3 and S2 subunits were substituted for each other. The mutant pro tein containing two S3 subunits (PTX Delta S2) presented a decreased bindin g to fetuin or haptoglobin but higher in vivo activity than the wild-type P TX, suggesting that replacement of S2 by S3 favours the targeting of PTX to the compartment where translocation occurs and/or the dissociation of S1 f rom the B oligomer, thereby leading to a better translocation of S1 into th e cytoplasm.