Expression of functionally active mammalian
histamine H1- and H2-receptors was recently demonstrated in Sf 9 cells. Either receptor elicited
phosphoinositide degradation leading to an increased cytoplasmic
calcium concentration. In the present study we focussed on identifying the Sf 9
guanine nucleotide-
binding proteins (
G proteins) involved. Immunodetection of Sf 9 membranes showed expression of G alpha
isoforms belonging to all four
G protein subfamilies. During prolonged baculovirus
infection of Sf 9 cells, binding of
guanosine 5'-o-(3-thiotriphosphate) as well as the intensities of
G protein immunoreactivity,
pertussis toxin-mediated ADP-ribosylation,
GTP azidoanilide labelling of G alpha, and
phosphate-labelling of G beta declined in cell membranes. Some 48 h after
infection with mammalian
histamine receptor-encoding viruses virtually no functional coupling of
ligand-activated receptors to insect
G proteins was observed despite a high level of expressed receptors. In contrast, Sf 9 cells infected only for 28 h allowed studies on
histamine-induced
G protein coupling. In membranes obtained from H1-receptor-expressing cells,
histamine increased incorporation of
GTP azidoanilide into Gq/11-like
proteins whereas in membranes containing H2-receptors
histamine enhanced
GTP azidoanilide-labelling of Gq/11-like and G(S)-like
proteins. In fura-loaded H1- and H2-receptor-expressing cells
histamine induced the release of
calcium from intracellular stores. This study shows firstly that Sf 9
G proteins couple to mammalian
histamine receptors and secondly that H1-receptors activate only Gq/11, whereas H2-receptors activate Gq/11 and G(S), but neither receptor couples to Gi/o or G12. Finally, the time following baculovirus
infection is critical for studying the functional coupling between recombinantly expressed and endogenous signal transduction components.