Antisense Homology Boxes in C5a Receptor and C5a Anaphylatoxin: A New Method for Identification of Potentially Active Peptides

A computer program was designed to locate regions termed antisense homology boxes (AHB), i.e., 8-15 amino acid regions corresponding to complementary DNA strands. Two AHBs were found in C5a and eight within the C5a receptor. The majority of intermolecular AHBs were found to overlap those found in the intramolecular AHBs. Several AHB peptides were synthesized and tested for their ability to interfere with C5a receptor functions. A peptide fragment of the C5a receptor corresponding to the loop between the fifth and sixth hypothetical transmembrane regions (amino acids 226-245) antisense to C5a and an intramolecular AHB in C5a receptor proved to be an antagonist of C5a when preincubated with C5a at high concentrations (>0.5 microM). However, when U937 cells bearing the C5a receptor were preincubated with this peptide at a much lower concentration (even as little as 40 pmol), the AHB peptide behaved as an agonist. Another AHB peptide corresponding to region 10-27 in the C5a receptor bound to two of its corresponding antisense peptides derived from C5a anaphylatoxin, corresponding to amino acids 37-43 and 61-74. This observation raises the possibility that the C5a receptor may bind C5a with two distinct orientations. Two other AHB peptides derived from C5a, PL12 (amino acids 12-27), and PL61 (amino acids 61-74), were also shown to inhibit activity. Incubation of dibutyryl cyclic AMP-stimulated U937 cells with PL37 (amino acids 37-51) resulted in increased intracellular Ca2+ levels and an anergy to subsequent challenge with C5a. Locating regions with sense-antisense relationships in proteins might help in identification of peptides that can interfere with the function of target proteins.