In neutrophils, activation of receptors for the chemotactic peptide N-formylmethionyl-leucyl-phenylalanine (fMLP) leads to changes in intracellular events such as phosphoinositide turnover and Ca2+ mobilization. Studies have shown that activation of the cloned fMLP receptor can also lead to inhibition of cyclic AMP (cAMP) accumulation [Lang, Boulay, Li and Wollheim (1993) EMBO J. 12, 2671-2679; Uhing, Gettys, Tomhave, Snyderman and Didsbury (1992) Biochem. Biophys. Res. Commun. 183, 1033-1039]. These responses are apparently mediated through pertussis toxin-sensitive Gi proteins. Since other chemotactic factor receptors can couple to multiple G proteins, we examined the ability of the fMLP receptor to utilize a pertussis toxin-insensitive G protein, Gz, in its signal transduction pathways. The human fMLP receptor was transiently expressed in 293 and Ltk- cells, and subsequently assayed for receptor-mediated inhibition of cAMP accumulation and stimulation of phosphoinositide-specific phospholipase C. In transfected 293 cells, fMLP inhibited choriogonadotropin-stimulated cAMP accumulation by 50% and the response could be abolished by pertussis toxin. Co-expression of the fMLP receptor with the alpha subunit of Gz rendered the fMLP response pertussis toxin-insensitive, indicating that the endogenous Gi proteins can be substituted efficiently by Gz. In contrast, Ltk- cells expressing the fMLP receptor were able to respond to fMLP with an increase in the production of inositol phosphates, but this response was completely abolished by pertussis toxin even in cells co-expressing the alpha subunit of Gz. Thus, although both signalling pathways appeared to utilize Gi-like proteins, Gz can only replace Gi in mediating inhibition of cAMP accumulation, and not in the stimulation of phospholipase C. Differential interaction with Gz might represent a novel mechanism by which fMLP receptors regulate intracellular events.

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