Structural insight into the evolution of a new chemokine family from zebrafish

The mammalian chemokine family is segregated into four families - CC, CXC, CX3C, and XC-based on the arrangement of cysteines and the corresponding disulfides. Sequencing of the Danio rerio (zebrafish) genome has identified more than double the amount of human chemokines with the absence of the CX3C family and the presence of a new family, CX. The only other family with a single cysteine in the N-terminal region is the XC family. Human lymphotactin (XCL1) has two interconverting structures due to dynamic changes that occur in the protein. Similar to an experiment with XCL1 that identified the two structural forms, we probed for multiple forms of zCXL1 using heparin affinity. The results suggest only a single form of CXL1 is present. We used sulfur-SAD phasing to determine the three-dimensional structure CXL1. Zebrafish CXL1 (zCXL1) has three disulfides that appear to be important for a stable structure. One disulfide is common to all chemokines except those that belong to the XC family, another is similar to a subset of CC chemokines containing three disulfides, but the third disulfide is unique to the CX family. We analyzed the electrostatic potential of the zCXL1 structure and identified the likely heparin-binding site for glycosaminoglycans (GAGs). zCXL1 has a similar sequence identity with human CCL5 and CXCL12, but the structure is more related to CCL5. Our structural analysis supports the phylogenetic and genomic studies on the evolution of the CXL family.