Divergent Traits and Ligand-Binding Features of the Cytomegalovirus CD48 Gene Family

The genesis of gene families through the capture of host genes and their subsequent duplication is a crucial process in the evolution of large DNA viruses. CD48 is a cell surface protein with an ectodomain composed of two immunoglobulin (Ig) domains. Via its N-terminal Ig domain, CD48 interacts with the cell surface receptor 2B4, triggering signal transduction events that regulate leukocyte cytotoxicity. We previously reported the presence of five CD48 homologs (vCD48s) in two related cytomegaloviruses, derived from a common host CD48 ancestor gene acquired by retrotranscription. Recently, we examined one member of this family, A43, showing that it acts as a functional viral decoy receptor, binding with high affinity and stability to 2B4 and impairing NK-cell cytotoxicity. Here, we have characterized the rest of the vCD48s. We show that they are highly glycosylated type I transmembrane proteins that display remarkably distinct features: dissimilar structures (e.g., different size stalks and cytoplasmic tails), biochemical properties, locations (cell surface/soluble), and temporal kinetic classes. We found that, in contrast to A43, none of them interacts with 2B4. Consistent with this, the molecular modeling of the N-terminal Ig domains of these vCD48s evidences significant changes in the numbers and lengths of their β-strands and inter-sheet loops that participate in the interaction of CD48 with 2B4. This suggests that these vCD48s have diverged to perform new 2B4-independent functions. Interestingly, we determined that one of them, S30, tightly binds CD2, a T- and NK-cell adhesion and costimulatory molecule whose primary ligand is CD58. Thus, altogether, our results show how a key host immune receptor captured by a virus can be subsequently remodeled during viral evolution to yield new molecules with improved affinities to their cognate receptors or with shifted binding specificities to additional immune targets, expanding the repertoire of viral immunoevasins.