Identification and Characterization of the Very-Low-Density Lipoprotein Receptor Gene from Branchiostoma belcheri : Insights into the Origin and Evolution of the Low-Density Lipoprotein Receptor Gene Family.

Low-density lipoprotein receptors (LDLRs) are a class of cell-surface endocytosis receptors that are mainly involved in cholesterol homeostasis and cellular signal transduction. Very-low-density lipoprotein receptors (VLDLRs), which are members of the LDLR family, have been regarded as multi-function receptors that fulfill diverse physiological functions. However, no gene has been identified in protochordates to date. As a representative protochordate species, amphioxi are the best available example of vertebrate ancestors. Identifying and characterizing the gene in amphioxi has high importance for exploring the evolutionary process of the LDLR family. With this study, a new amphioxus gene (designated ) was cloned and characterized using RACE-PCR. The 3217 nt transcript of the had a 2577 nt ORF, and the deduced 858 amino acids were highly conserved within vertebrate VLDLRs according to their primary structure and three-dimensional structure, both of which contained five characteristic domains. In contrast to other vertebrate VLDLRs, which had a conserved genomic structure organization with 19 exons and 18 introns, the had 13 exons and 12 introns. The results of a selective pressure analysis showed that the had numerous positive selection sites. Furthermore, the tissue expression of using RT-qPCR showed that RNA expression levels were highest in the gills and muscles, moderate in the hepatic cecum and gonads, and lowest in the intestines. The results of the evolutionary analysis demonstrated that the gene is a new member of the VLDLR family whose family members have experienced duplications and deletions over the evolutionary process. These results imply that the functions of LDLR family members have also undergone differentiation. In summary, we found a new gene homolog () in amphioxi. Our results provide insight into the function and evolution of the gene family.