Exploring platelet chemokine antimicrobial activity: nuclear magnetic resonance backbone dynamics of NAP-2 and TC-1

The platelet chemokines neutrophil-activating peptide-2 (NAP-2) and thrombocidin-1 (TC-1) differ by only two amino acids at their carboxy-terminal ends. Nevertheless, they display a significant difference in their direct antimicrobial activities, with the longer NAP-2 being inactive and TC-1 being active. In an attempt to rationalize this difference in activity, we studied the structure and the dynamics of both proteins by nuclear magnetic resonance (NMR) spectroscopy. Using 15N isotope-labeled protein, we confirmed that the two monomeric proteins essentially have the same overall structure in aqueous solution. However, NMR relaxation measurements provided evidence that the negatively charged carboxy-terminal residues of NAP-2 experience a restricted motion, whereas the carboxy-terminal end of TC-1 moves in an unrestricted manner. The same behavior was also seen in molecular dynamic simulations of both proteins. Detailed analysis of the protein motions through model-free analysis, as well as a determination of their overall correlation times, provided evidence for the existence of a monomer-dimer equilibrium in solution, which seemed to be more prevalent for TC-1. This finding was supported by diffusion NMR experiments. Dimerization generates a larger cationic surface area that would increase the antimicrobial activities of these chemokines. Moreover, these data also show that the negatively charged carboxy-terminal end of NAP-2 (which is absent in TC-1) folds back over part of the positively charged helical region of the protein and, in doing so, interferes with the direct antimicrobial activity.