:the pertussis toxin was found to interfere with the ligand-binding site in g-protein coupled receptors. at which point in the cell signaling pathway would this toxin interrupt normal cell signaling

The point at which the toxin would interrupt normal cell signalling in the pathway is the signal amplification. 
This is because of the G-protein uncoupling and inhibition of signal amplification by pertusis toxin. Pertusis toxin released by the bacteria Bordetella pertusis and prevents signal that is amplifying from the protein. The G-protein coordinates the interaction between membrane bound receptor proteins and the effector proteins involved in the intracellular signalling. The toxin promotes the uncoupling of this heterotrimetric protein and also inhibits the amplification thus preventing the interaction of the receptor proteins and the second messengers.