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We set out to identify potential key regulators of exocytotic fusion in the eosinophil, in the knowledge that granule exocytosis can be stimulated in these cells by intracellular application of nonhydrolyzable analogues of guanosine triphosphate, with Ca2+ acting as a modulator of guanine nucleotide-dependent secretion. To screen for GTP-binding proteins, guinea pig eosinophils were purified from peritoneal washings and subjected to western blotting analysis using specific immune sera raised against recombinant proteins or consensus peptide sequences within proteins of interest. We found a number of heterotrimeric G proteins (G alpha i3, G alpha o, G alpha q11, G alpha s and G beta subunits) and members of the small GTP-binding proteins expressed in eosinophils. Two subtypes of G-protein alpha subunits (G alpha i1 and G alpha z) could not be detected. Separation of subcellular organelles from homogenized eosinophils by density gradient centrifugation revealed that all of the detected GTP-binding proteins were mainly expressed in fractions containing peak plasma membrane and Golgi marker enzyme activities, while G beta subunits were also detected in secretory granule fractions. However, isoforms of Rab3, a putative GTP-binding regulator of exocytotic fusion, were undetectable in eosinophils. Neither, with the exception of syntaxin-3, could we detect any of the proteins belonging to the proposed synaptic vesicle fusion complex (SNAP-25; synaptobrevin (VAMP) and its non-neuronal homologue, cellubrevin; synaptophysin; synaptotagmin). The results from this study, based on western blotting, suggest that eosinophils express a different class of exocytotic fusion complex proteins from those found in neuronal tissues, although a number of potential candidates fulfilling the role of GE were identified in this important inflammatory cell.