A search within the IL-1 type I receptor reveals a peptide with hydropathic complementarity to the IL-1β trigger loop which binds to IL-1 and inhibits in vitro responses

In previous research, we were able to demonstrate that a seven amino acid residue peptide (VITFFSL), designed as an antisense peptide of the β-bulge trigger loop region of interleukin 1β (IL-1β) (QGEESND; residues 48–54 [mature protein sequence]), was able to interact with IL-1 specifically and inhibit the response to IL-1 in an in vitro bioassay. The evidence was consistent with a specific interaction ocurring between antisense peptide and the trigger loop region. On the basis that antisense peptides are able to interact with their corresponding sense peptide sequences as a result of their mutually complementary hydropathic profiles (Fassina G., Verdoliva, A., Cassani, G., Melli, M., 1994. Binding of type I IL-1 receptor fragment 151–162 to IL-1. Growth Factors 10, 99–106; Maier, C.C., Moseley, H.N.B., Zhou, S., Whitaker, J.N., Blalock, J.E., 1994. Indentification of interactive determinants on idiotypic-anti-idiotypic antibodies through comparison of their hydropathic profiles. Immunomethods 5, 107–113), we devised a computer program (FINDH) to search the amino acid residue sequence of interleukin-1 type 1 receptor (IL-1 R1) for peptide motifs possessing hydropathic complementarity to the trigger loop sequence. The most complementary “best-fit peptide” motif (LITVLNI) was located in the third extracellular domain of IL-1 R1. A best-fit peptide corresponding to this motif was synthesised and found to bind to IL-1β as well as inhibit the response to IL-1 in two independent in vitro bioassays (monitoring IL-1 dependent serum amyloid A synthesis and IL-1 dependent alkaline phosphatase activity, respectively). A second peptide motif (VIEFITL) was identified and the corresponding peptide synthesised along with a reordered version (LTILINV) of the best fit peptide. Both failed to bind measurably with IL-1β or inhibit the response to IL-1 in the two bioassays. This best fit peptide behaved very similarly, in terms of IL-1 binding and inhibition behaviour, to the original trigger loop antisense peptide. Reference to the recently released X-ray crystal structure of IL-1β and the IL1-R1 extracellular domain shows that the best fit peptide motif in IL-1 R1 is not apparantly interacting with the IL-1 trigger loop, although both are close in space. The intriguing possibility exists that the best fit peptide motif could represent an alternative site for IL-1β receptor interaction which has not thus far been identified.