Mutations in the S1 subunit of pertussis toxin that affect secretion

Pertussis toxin is a member of the AB(5) family of toxins and is composed o f five subunits (S1 to S5) present in a 1:1:1:2:1 ratio. Secretion is a com plex process. Each subunit has a secretion signal that mediates transport t o the periplasm, where processing and assembly occur. Secretion of the asse mbled 105-kDa toxin past the outer membrane is mediated by the nine protein s encoded in the ptl operon. Previous studies have shown that S1, the catal ytically active A subunit of pertussis toxin, is necessary for efficient se cretion, suggesting that a domain on S1 may be required for interaction wit h the secretion apparatus. Previously, recombinant S1 from four different m utants (serine 54 to glycine, serine 55 to glycine, serine 56 to glycine, a nd arginine 57 to lysine) was shown to retain catalytic activity. We introd uced these mutations into Bordetella pertussis and monitored pertussis toxi n production and secretion. No pertussis toxin was detected in the serine 5 4-to-glycine mutant. The other SI mutants produced periplasmic pertussis to xin, but little pertussis toxin secretion was observed, The arginine 57-to- lysine mutant had the most dramatic secretion defect. It produced wild-type levels of periplasmic pertussis toxin but secreted only 8% as much toxin a s the wild-type strain. This phenotype was similar to that observed for str ains with mutations in the ptl genes, suggesting that this region may have It role in pertussis toxin secretion.