Backbone dynamics of SDF-1α determined by NMR:: Interpretation in the presence of monomer-dimer equilibrium

SDF-1 alpha is a member of the chemokine family implicated in various reactions in the immune system. The interaction of SDF-1 alpha with its receptor, CXCR4, is responsible for metastasis of a variety of cancers. SDF-1 alpha is also known to play a role in HIV-1 pathogenesis. The structures of SDF-1 alpha determined by NMR spectroscopy have been shown to be monomeric while X-ray structures are dimeric. Biochemical data and in vivo studies suggest that dimerization is likely to be important for the function of chemokines. We report here the dynamics of SDF-1 alpha determined through measurement of main chain N-15 NMR relaxation data. The data were obtained at several concentrations of SDF-1 alpha and used to determine a dimerization constant of similar to 5 mM for a monomer-dimer equilibrium. The dimerization constant was subsequently used to extrapolate values for the relaxation data corresponding to monomeric SDF-1 alpha. The experimental relaxation data and the extrapolated data for monomeric SDF-1 alpha were analyzed using the model free approach. The model free analysis indicated that SDF-1 alpha is rigid on the nano- to picosecond timescale with flexible termini. Several residues involved in the dimer interface display slow micro- to millisecond timescale motions attributable to chemical exchange such as monomer-dimer equilibrium. NMR relaxation measurements are shown to be applicable for studying oligomerization processes such as the dimerization of SDF-1 alpha.