Lymphotactin (Ltn) is a unique chemokine that under physiological solution conditions displays large-scale structural heterogeneity, defining a new category of ‘metamorphic proteins’. Previous Ltn studies have indicated that each form is required for proper function, but the mechanism of interconversion remains unknown. Here we have investigated the temperature dependence of kinetic rates associated with interconversion and unfolding by stopped-flow fluorescence to determine transition-state free energies. Comparisons of derived thermodynamic parameters revealed striking similarities between interconversion and protein unfolding. We conclude that Ltn native state rearrangement proceeds by way of a large-scale unfolding process rather than a unique intermediate structure.

Background: GAG/CXCL12 interactions are critical for chemokine function but co-administration may abrogate their individual cardioprotective effects in a clinical setting. Results: Biophysical studies distinguish CXCL12 residues involved in dimerization from those likely to contact heparin directly. Conclusion: CXCL12 dimerization is required for high affinity heparin binding and protects N-terminal degradation. Significance: Knowledge of the GAG-binding site will enable future development of heparin-insensitive CXCL12 therapeutics.