G protein-mediated pathways are fundamental mechanisms of cell signaling. In this paper, the expression and the characterization of the alpha(il), a(i3), alpha(o1), beta(1), and gamma(2) subunits of the human G protein are described. This approach was developed to evaluate the G protein activation profile of new compounds. pCR-TOPO T7 vectors, engineered to contain the target sequences, were used to transform Escherichia coli competent cells. Subunits were over-expressed in a preparative scale as fusion proteins with a six-histidine tag, and subsequently purified by metal chelate chromatography. Afterward, the His-tag was removed by enterokinase digestion, and the secondary structures of the recombinant subunits were analyzed by circular dichroism. To assess the functionality of the subunits, the rate of GTP hydrolysis and GTP gamma S binding were evaluated both in the absence and in the presence of two modulators: the peptidic activator Mastoparan and the non-peptidic activator N-dodecyl-lysinamide (ML250). Tests were conducted on isolated alpha-subunit and on heterotrimeric alpha beta gamma complex, alone or reconstituted in phospholipidic vesicles. Our results show that recombinant subunits are stable, properly folded and, fully active, which makes them suitable candidates for functional studies. (c) 2005 Elsevier Inc. All rights reserved.