Variable Lymphocyte Receptor-like Proteins From Amphioxus

The immunity systems are grouped into innate immunity system and adaptive immunity system(AIS).Innate immunity system is conserved from insects to mammals that utilizes pattern recognition receptors(PRRs)to recognize highly conserved pathogen-associated molecular patterns(PAMPs).AIS is generally believed to only exist in vertebrates.Recent studies have shown that vertebrates evolve two alternative adaptive immune systems,one in jawed vertebrates,from cartilaginous fish to mammals,and employ BCRs/TCRs as antigen receptors to activate immune response,whereas another one which exists only in jawless vertebrates uses variable lymphocyte receptors(VLRs)that are composed of leucine-rich repeat(LRR)modules to recognize antigens.Both AISs generate a large receptor repertoire of sufficient diversity to recognize various antigenic components of potential pathogens or toxins.Discovery of VLR in jawless vertebrate has brought the origin of AIS forward to 500 million years accompanying with the emergence of vertebrates.Previous findings indicated that chordate amphioxus,which is an important model animal for the study of vertebrates,also possesses some homologs of the basic components of TCR/BCR-based AIS,but it remains unknown if there exist any components of VLR-based AIS in amphioxus.Bioinformatic analyses revealed the amphioxus Branchiostoma floridae encodes a group of putative VLR-like proteins.Here we reported 1.79 A crystal structure of Bf66946,which forms a crescent-shaped structure of five LRRs.Structural comparisons indicated that Bf66946 resembles the lamprey VLRC.Further electrostatic potential analyses showed a negatively-charged patch at the concave of LRR solenoid structure that might be responsible for antigen recognition.Site-directed mutagenesis combined with bacterial binding assays revealed that Bf66946 binds to the surface of Gram-positive bacteria Staphylococcus aureus and Streptococcus pneumonia via a couple of acidic residues at the concave.In addition,the closest homolog of Bf66946 is highly expressed in the potential immune organ gill of Branchiostoma belcheri,implying Bf66946 is an immune related protein.All together,we identified the first VLR-like protein and our findings provide the first structural evidence for the emergence of VLR-like molecules in the basal chordates amphioxus,showing insight into the origin and evolution of AIS.