What is the function of the S1 subunit in PTx?

• A. ADP ribosyl transferase
• B. Receptor binding
• C. Immune response modulation
• D. Metabolic regulation

Answer: (A)

The S1 subunit of PTx, or pertussis toxin, functions as an ADP ribosyl transferase. This means it is responsible for transferring an ADP-ribose moiety from NAD+ to a target protein, typically a G-protein in host cells. This modification leads to the inhibition of the G-protein's normal function, which is critical for various signaling pathways within the host. By inhibiting these pathways, S1 disrupts normal cellular processes, effectively contributing to the pathogenic effects of the toxin.

The other roles mentioned in the options are not associated with the primary biochemical role of the S1 subunit in PTx. While PTx does affect immune responses and may have indirect effects on metabolic regulation and receptor binding through the actions of the complete toxin, these processes stem from the ADP ribosylation activity of S1. Thus, the ADP ribosyltransferase function is central to the mechanism of action of PTx.