Our previous study suggested that the region encompassing residues 220-240 on G(alpha 16) is important in coupling with C5a receptor (Lee et al. (1995) Mel. Pharmacol. 47, 218-223). When aligned sequences are compared in the residue 220-240 segment of G(alpha 16), there is a block of eight amino acids extending from residue 227 to residue 234 (227-Ile-Ala-Leu-Ile-Tyr-Leu-AlaSer-234) in G(alpha 16) that is replaced by a heterologous block extending from amino acid residue 224 to residue 231 (224-Thr-Ser-Ile-Met-Phe-Leu-Val-Ala-231) in G(alpha 11). In order to identify the specific amino acid residue necessary for coupling to C5a receptor within the extension of eight amino acids in G(alpha 16), a series of chimeric G(alpha 11)/G(alpha 16) cDNA constructs and mutant G(alpha 16) cDNAs were expressed. Then the ability of chimeras and mutant proteins to mediate C5a-induced release of inositol phosphate in transfected Cos-7 cells was tested. The results show that single amino acid Ala(228) is responsible for conferring about 40-50% of the activity of G(alpha 16) induced by C5a receptor stimulation.