Expression, activity and cytotoxicity of pertussis toxin S1 subunit in transfected mammalian cells

Pertussis toxin (PT) comprises an active subunit (S1), which ADP-ribosylate s the alpha subunit of several mammalian G proteins, and the B oligomer (S2 -S5), which binds glycoconjugate receptors on cells. In a previous report, expression of S1 in Cos cells resulted in no observable cytotoxicity, and i t was hypothesized that either S1 failed to locate its target proteins or t he B oligomer was also necessary for cytotoxicity. To address this, we stab ly transfected S1 with and without a signal peptide into mammalian cells. I mmunofluorescence analysis confirmed the function of the signal peptide. Su rprisingly, we found that S1 was active in both transfectants, as determine d by clustering of transfected Chinese hamster ovary (CHO) cells and ADP-ri bosylation of G proteins. Constructs with a cysteine-to-serine change at re sidue 201 or a truncated S1 (residues 1-181) were also active when transfec ted into cells. Constructs with an inactive mutant S1 had no activity, conf irming that the observed results were due to the activity of the toxin subu nit. We conclude that S1 is active when expressed in mammalian cells withou t the B oligomer, that secretion into the endoplasmic reticulum does not pr event this activity and that the C-terminal portion of S1 is not required f or its activity in cells.