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RC Tsu, RA Allen and YH Wong
Department of Biology, Hong Kong University of Science and Technology, Kowloon.
The capacity of N-formylmethionyl-leucyl-phenylalanine (fMLP) and C5a receptors to regulate type II adenylyl cyclase was examined in transient transfection studies. Coexpression of either one of the chemoattractant receptors with type II adenylyl cyclase in human embryonic kidney 293 cells allowed the corresponding chemotactic factor to stimulate cAMP accumulation in a dose-dependent manner. The chemoattractant-induced stimulation of type II adenylyl cyclase was absolutely dependent on the presence of GTP-bound alpha subunit of GS, as revealed by the coexpression of alpha s-Q227L, a constitutively activated mutant of alpha s. Stimulation of type II adenylyl cyclase by either fMLP or C5a was mediated via pertussis toxin-sensitive Gi-like proteins, because the response was abrogated by the toxin. The ability of Gz (a pertussis toxin-insensitive G protein that can couple to a number of Gi-linked receptors) to replace Gi in chemoattractant-induced stimulation of type II adenylyl cyclase was examined. The chemoattractant-induced response became insensitive to pertussis toxin upon coexpression of the alpha subunit of Gz. Interestingly, coexpression of alpha z significantly enhanced the chemotactic factor- stimulated type II adenylyl cyclase activities. When other G protein alpha subunits were tested under similar experimental conditions, all three forms of alpha 1 and alpha o1 were able to potentiate the fMLP response to various extents, whereas alpha q and alpha t slightly inhibited the fMLP response. The alpha subunit-mediated potentiation of the type II adenylyl cyclase response appears to reflect a productive coupling between alpha subunits and the fMLP receptor, because such enhancements were not seen with the constitutively activated alpha subunit mutants. Coexpression of the constitutively activated mutants of alpha z, alpha q, alpha 01, and alpha i1-3 neither enhanced nor inhibited the fMLP-stimulated cAMP accumulation. These results indicated that the observed enhancement of type II adenylyl cyclase responses was dependent on the ability of the wild-type alpha subunits to functionally interact with the fMLP receptor and that the fMLP receptor can couple to Gi1-3, Gz, and Go1 but not to Gs, Gq, or Gt.
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