The heterodimeric complex of MRP-8 (S100A8) and MRP-14 (S100A9) - Antibody recognition, epitope definition and the implications for structure

The S100 calcium-binding proteins MRP-8 (S100A8) and MRP-14 (S100A9) form a heterodimeric complex in the cytosol of monocyte and neutrophil cell types circulating in peripheral blood. This complex, but not the individual subunit proteins, is specifically recognized by mAb 27E10. Domains in MRP-8 and MRP-14 mediating heterodimeric complex formation have not yet been identified but it is predicted that the structure of the complex will be similar to homodimeric forms of other S100 proteins. This study makes use of the specificity of mAb 27E10, and an in vitro coupled transcription/translation system to further examine the formation and maintenance of the MRP-8/MRP-14 complex. Truncated mutants of MRP-14 that lack the N-terminal residues 1-4 or the extended C-terminal 'tail', both complex with MRP-8. These deleted domains of MRP-14 are therefore not essential for complex formation. Peptides from MRP-8 or MRP-14, used to induce the epitope recognized by mAb 27E10, show that a critical interaction in complex formation involves the N-terminal of MRP-8 interacting with MRP-14. Phage display analysis defined composite residues of the epitope recognized by mAb 27E10. The epitope is trans-subunit, composed of residues in the C-terminal ends of helix IV in MRP-14 and helix I of MRP-8. A further complex-specific mAb, named 5.5, recognizes the hydrophobic residues in helix IV of MRP-8, exposed during heterodimer formation. The definition of these two epitopes indicates that helices IV of MRP-8 and MRP-14 are also a prominent point of interaction and suggests that the subunit proteins will assume an antiparallel alignment in the heterodimer, similar in structure to the homodimeric forms of S100 proteins.