Correspondence of the functional epitopes of poxvirus and human interleukin-18-binding proteins.
Molluscum contagiosum virus, a human poxvirus that causes persistent small benign skin tumors, encodes a variety of putative immune defense proteins. Three such proteins, MC51L, MC53L, and MC54L, have 20 to 35% amino acid sequence identities with human interleukin-18 (hIL-18)-binding protein (hIL-18BP), a naturally occurring antagonist of the proinflammatory cytokine IL-18. We previously demonstrated that seven amino acids within the immunoglobulin-like domain of hIL-18BP were important for high-affinity binding to hIL-18. Model building indicated that MC54L, which has been shown to bind hIL-18, contains five of the seven amino acids at corresponding positions in its immunoglobulin-like domain, the exceptions being the conservative substitution of isoleucine for a leucine and the nonconservative substitution of valine for a phenylalanine. We found that individual alanine substitutions for these six identical or highly conserved amino acids of MC54L caused changes in affinity and binding free energy for hIL-18 that were quantitatively similar to those produced by mutagenesis of hIL-18BP. Furthermore, when the nonconserved valine of MC54L was mutated to phenylalanine, making it more like hIL-18BP, its affinity for hIL-18 increased more than 10-fold. In addition, the carboxyl-terminal half of MC54L, which has no similarity with hIL-18BP, was dispensable for hIL-18 binding. Thus, despite their relatively low overall sequence identity, MC54L and hIL-18BP have similar hIL-18 binding sites and functional epitopes. On the other hand, MC51L and MC53L have nonconservative substitutions of three to six of the seven critical amino acids of hIL-18BP and neither protein bound hIL-18, suggesting that they may interact with unidentified ligands.
Xiang
Y
Y
Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892-0445,USA.
Moss
B
B
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001665:Binding Sites; D000939:Epitopes / Q000737:chemistry / Q000235:genetics; D006023:Glycoproteins / Q000737:chemistry*; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D008977:Molluscum contagiosum virus / Q000737:chemistry*; D009154:Mutation; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry; D016415:Sequence Alignment; D014633:Valine / Q000737:chemistry; D014764:Viral Proteins / Q000737:chemistry* / Q000235:genetics
D000939:Epitopes; D006023:Glycoproteins; D036341:Intercellular Signaling Peptides and Proteins; D011994:Recombinant Proteins; D014764:Viral Proteins; C118370:interleukin-18 binding protein; D014633:Valine