The N-formyl peptide receptor (FPR) of human neutrophils is a member of the G protein-coupled receptor (GPCR) superfamily. Sites on agonist-occupied FPR involved in binding the Gi2 protein were investigated by competition with synthetic receptor-mimetic peptides. Twenty-three synthetic FPR-mimetic and control peptides were tested for their ability to disrupt functionally active complexes of FPR and Gi2 in octyl glucoside, assayed by changes in sedimentation rates of FPR in detergent-containing sucrose gradients. GPCRs are thought to contain seven transmembrane segments with three cytoplasmic connecting loops and a cytoplasmic tail. Only certain peptides from regions in or adjoining each of the four predicted cytoplasmic domains of the 350 amino acid FPR, including the first cytoplasmic loop, were able to disrupt the reconstituted FPR-Gi2 complex. The IC50s of the peptides that were able to fully disrupt the FPR-Gi2 complex ranged from 20 microM (C2W 122-134) to 1.4 mM (C3A 230-246), a range similar to peptide inhibition of other G protein-coupled receptor-G protein interactions. Detergent concentrations above and below the critical micelle concentration had no effect on the activity of even the most hydrophobic peptide, C3B, and there was no apparent correlation of activity with hydrophobic moment, hydrophilic index, or net charge of the peptides. Control peptides from irrelevant proteins with similar physical properties and FPR extracellular domains did not dissociate the reconstituted FPR-Gi2 complex up to 5 mM, the highest concentration tested.(ABSTRACT TRUNCATED AT 250 WORDS)