Cloning, expression, and characterization of fugu CD4, the first ectothermic animal CD4

We have cloned and sequenced the first ectothermic animal CD4 gene from fugu, Takifugu rubripes, using a public database of the third draft sequence of the fugu genome. The fugu CD4 gene encodes a predicted protein of 463 amino acids containing four extracellular immunoglobulin (Ig)-like domains, a transmembrane region, and a cytoplasmic tail. Fugu CD4 shares low identity of about 15-20% with avian and mammalian CD4 proteins. Unlike avian and mammalian CD4, fugu CD4 lacks the Cys pair of the first Ig-like domain, but has a unique possible disulfide bond in the third domain. These differences suggest that fugu CD4 may have a different structure that could affect binding of major histocompatibility complex class II molecules and subsequent T-cell activation. In the putative fugu cytoplasmic region, the protein tyrosine kinase p56lck binding motif is conserved. The predicted fugu CD4 gene is composed of 12 exons, differing from other CD4 genes, but showing conserved synteny and many conserved sequence motifs in the promoter region. RT-PCR analysis demonstrated that the fugu CD4 gene is expressed predominantly in lymphoid tissues. We also show that fugu CD4 can be expressed on the surface of cells via transfection. Molecular characterization of CD4 in fish provides insights into the evolution of both the CD4 molecule and the immune system.