Absence of G(i) proteins in the Sf9 insect cell. Characterization of the uncoupled recombinant N-formyl peptide receptor.

We investigated the interaction of the N-formyl peptide receptor (NFPR) with G proteins in infected Sf9 insect cells expressing the recombinant NFPR. Recombinant receptor expression of up to 27 pmol/mg protein was achieved in these cells. The receptor was recognized by an antiserum raised against an NFPR carboxyl-terminal peptide, and displayed specific and saturable binding of the formyl peptide ligand fMet-Leu-[3H]Phe. Scatchard analysis of the binding data yielded a dissociation constant of approximately 62 nM, a binding affinity of 60- to 120-fold lower than that of the high affinity sites in neutrophils and in transfected mammalian cell lines expressing the NFPR. That this low binding affinity was due to a lack of receptor coupling to G protein was suggested by the failure of guanine nucleotides to regulate receptor affinity and by the lack of formyl peptide-stimulated GTPase activity in these cells. Furthermore, immunoblotting with an anti-G(i) antibody and ADP-ribosylation experiments indicated that the approximately 40-kDa G(i) alpha subunit, which couples to the NFPR in neutrophils, is not present in Sf9 cell membranes. Thus, the current study provides for the first time evidence that a major G protein is absent in the Sf9 insect cells. Potential applications of the Sf9 system for in vitro reconstitution of the NFPR-G protein interaction are discussed.