Is sequence conservation in interferons due to selection for functional proteins?

Abstract

The human α-interferon (IFN-α) gene family consists of at least 14 potentially functional non-allelic members; the amino acid sequences they encode differ from each other by up to ∼20% of their residues (ref. 1 and K. Henco et al., in preparation). Human IFN-β, which is encoded by a single gene, is distantly related to the IFN-α family₂; it differs in 67% of its residues from IFN-α₂. There is considerable evidence that IFN-α and -β compete for the same receptors on their target cells^3–6. Comparison of 14 non-allelic human IFN-α sequences and the IFN-β sequence has revealed that 37 of 166 residues are completely conserved and that several of these are arranged in clusters, for example at positions 29–33, 47–50 and 136–150. It is commonly held (for example, refs 2, 7) that evolutionary conservation of amino acids indicates that the residues in question are essential for function. To test this hypothesis in the case of IFNs, we have introduced single site-directed point mutations into the strictly conserved codons 48 and 49 of the IFN-α₂ gene which form part of the longest uninterrupted cluster (position 47–50). We report here that the mutant proteins, containing Tyr, Ser and Cys instead of Phe48, or His instead of Gln49, have biological activities indistinguishable from those of wild-type IFN-α. In addition, when Glu62, a residue conserved in all known α and β IFNs of man, mouse and cattle, was replaced by Lys, antiviral activity remained unchanged.