NMR reveals an ordered but conformationally exchanging N-terminus in the chemokine CCL11

The C-C motif Chemokine Ligand 11 (CCL11) is a promiscuous messenger for multiple C-C motif G protein-coupled receptors in the human immune system. Previously solved structures, including CCL11 bound to a truncated CCR3 peptide and apo-CCR3 bound to the heterotrimeric G protein, provide clues to the initiation of ligand-receptor interaction; however, a consensus on the full-length interaction has yet to be reached. Here, we present a structure of CCL11 solved utilizing liquid-state Nuclear Magnetic Resonance (NMR) data in buffered solution at pH 5.0 with 100 mM NaCl. The resulting ensemble exhibits not only secondary structural elements defined differently from past reports, but also a less dynamic N-terminus, showing local exchange correlated with the 30s-loop. Fast (ns) and intermediate (µs-ms) dynamics were measured via a combination of relaxation techniques, revealing that most N-terminal and 30s-loop residues exchange, while only a few residues at the N-terminal tip move freely. We hypothesize that this local exchange contributes to induced binding, and that N-terminal and 30s-loop dynamics could be related to chemokine promiscuity. Rigid-body docking to an AlphaFold2 CCR3 model generated candidate geometries in which the CCL11 N-terminus reaches orthosteric pocket residues previously implicated in the literature. In total, our results highlight that dynamics captured on two distinct timescales together show that the N-terminus of CCL11 is ordered as a tail but undergoes conformational exchange linked to the 30s-loop.