The use of membrane translocating peptides to identify sites of interaction between the C5a receptor and downstream effector proteins.

The complement fragment C5a is a potent leucocyte chemoattractant and activator, mediating its effects through a G-protein-coupled receptor. Whilst the C-terminal domain of this receptor has been shown to be essential for receptor desensitization and internalization, it is not known which domains couple to the receptor's heterotrimeric G proteins. In this report we have used a membrane translocating sequence (MTS) to examine the effects of the four intracellular domains of the human C5a receptor (C5aR) on the receptor's signalling via G(alphai) family heterotrimeric G proteins in intact RBL-2H3 cells. The results indicate that all of the intracellular domains couple to downstream signalling, with the proximal region of the C terminus being a major binding site and intracellular loop 3 playing a role in G protein activation or receptor desensitization.