Chemokine CXCL1, and its receptor CXCR2, a class-A G protein-coupled receptor (GPCR), play a crucial role in directing blood neutrophils to sites of infection and injury. A dysregulation in CXCR2 activation results in host tissue damage and disease. CXCL1 binds CXCR2 at two distinct sites: N- terminal domain (Site-I, unique to chemokines) and a groove defined by the receptor extracellular loops/transmembrane helices (Site-II, shared with all class A receptors). The molecular basis by which chemokine binding two distinct sites determine CXCR2 activation is not known. Structures and sequence analyses reveal that chemokine and receptor residues that mediate binding are either unstructured or conformationally dynamic. We propose that CXCL1 binding at Site-I of CXCR2, the initial obligatory step, triggers structural and dynamic changes that are essential for Site-II interactions. We will test our hypothesis using a hybrid strategy that combines nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. We will determine the structure and characterize the role of conformational dynamics of Site-I CXCL1-CXCR2 N-terminal domain complex using NMR spectroscopy (Aim 1). We will generate a structural model of CXCL1 bound to CXCR2 at the N-terminal domain by merging Site-I NMR structure and previously determined CXCR2 structure, and use extended MD simulations to describe how CXCL1 bound at Site-I engages the receptor at Site-II (Aim 2). We will characterize how CXCL1 Site-I and Site-II residues identified from Aims 1 and 2 mediate CXCR2 activation using cellular assays (Aim 3). These studies will provide critical insights into the molecular mechanisms underlying CXCR2 activation and will advance designing therapeutics that disrupt CXCR2 activation and alleviate disease.