Evolution of a Metamorphic Protein

Abstract

Metamorphic proteins adopt two or more different native folds, expanding the protein folding paradigm. Lympotactin/XCL1 is a metamorphic chemokine that spontaneously unfolds and reversibly interconverts between two distinct native structures (an all beta-sheet dimer and the alpha-beta chemokine fold). Like most chemokines, XCL1 stimulates immune cell migration by activating its G protein-coupled receptor, XCR1, and binding to extracellular matrix glycosaminoglycans. We investigated the emergence of XCL1's metamorphic folding using ancestral sequence reconstruction, biophysical characterization, and bioinformatic analysis. The results suggest that divergence of XCL1 from its chemokine paralogs altered the folding energy landscape in order to populate its two native states in approximately equal proportion. In addition to its functional role in the adaptive immune system, XCL1 also exhibits antimicrobial activity, analogous to other antimicrobial peptides and chemokines. We speculate that non-canonical roles in innate immune function contributed to the emergence of metamorphic folding exhibited by extant human XCL1 protein.