Multiple regions of G alpha 16 contribute to the specificity of activation by the C5a receptor.

The C5a chemoattractant factor receptor, when expressed in COS-7 cells, can stimulate phosphoinositide-specific phospholipase C activity through the activation of the G16 isoform of the heterotrimeric G protein, but not through the G11 isoform. To identify the regions of the G alpha 16 subunit protein that are responsible for its activation by the C5a receptor, a series of chimeras between G alpha 16 and G alpha 11 were constructed and tested for their ability to be activated by the C5a receptor. Co-transfection experiments with chimeras in which the carboxyl-terminal regions of G alpha 11 were replaced with the corresponding regions of G alpha 16 demonstrated that changes in the carboxyl terminus, e.g., replacement of 134 amino acids, were not sufficient to confer receptor specificity. An additional segment encompassing residues 220-240 of G alpha 16 was required to confer C5a-induced activation. Testing of a reciprocal series of chimeras composed of G alpha 16 sequences at the amino terminus and G alpha 11 sequences at the carboxyl terminus revealed that certain sequences extending from the amino terminus to amino acid 209 of G alpha 16 were sufficient to endow the chimera with much of the specificity for C5a-induced activation. These results suggest that receptor specificity may involve specific conformations of the G protein stabilized by concerted interactions of multiple amino acid sequences distributed throughout the G alpha protein.