Functional characterization of monoclonal antibodies to interferon-tau.

Interferon tau (IFNtau) produces an array of biological effects, including antiluteolytic, antiviral, antiproliferative and immunomodulatory activities, without the consequent cytotoxicity associated with other type I IFNs. Four anti-IFNtau monoclonal antibodies (MAbs) have been characterized by determining regional epitopes and observation of their effects on IFNtau binding, antiviral and antiproliferative activity. Using an enzyme-linked immunoadsorbent assay (ELISA) developed against six overlapping synthetic peptides representing the entire linear sequence of IFNtau, three antibodies, HL-98, HL-100 and HL-127, were found to react with the carboxy terminal peptide, while HL-129 bound the penultimate amino terminal peptide. Binding studies indicated that MAbs directed against either region could effectively inhibit the binding of alkaline phosphatase labeled IFNtau to cells expressing type I IFN receptors. While only two of the MAbs significantly reversed IFNtau-induced growth inhibition, the antiviral activity of IFNtau was significantly inhibited by MAbs that bound the amino and carboxy termini, confirming the functional importance of these domains in the binding and subsequent activity of IFNtau.

Critical aspartic acid residues in pseudouridine synthases.

The pseudouridine synthases catalyze the isomerization of uridine to pseudouridine at particular positions in certain RNA molecules. Genomic data base searches and sequence alignments using the first four identified pseudouridine synthases led Koonin (Koonin, E. V. (1996) Nucleic Acids Res. 24, 2411-2415) and, independently, Santi and co-workers (Gustafsson, C., Reid, R., Greene, P. J., and Santi, D. V. (1996) Nucleic Acids Res. 24, 3756-3762) to group this class of enzyme into four families, which display no statistically significant global sequence similarity to each other. Upon further scrutiny (Huang, H. L., Pookanjanatavip, M., Gu, X. G., and Santi, D. V. (1998) Biochemistry 37, 344-351), the Santi group discovered that a single aspartic acid residue is the only amino acid present in all of the aligned sequences; they then demonstrated that this aspartic acid residue is catalytically essential in one pseudouridine synthase. To test the functional significance of the sequence alignments in light of the global dissimilarity between the pseudouridine synthase families, we changed the aspartic acid residue in representatives of two additional families to both alanine and cysteine: the mutant enzymes are catalytically inactive but retain the ability to bind tRNA substrate. We have also verified that the mutant enzymes do not release uracil from the substrate at a rate significant relative to turnover by the wild-type pseudouridine synthases. Our results clearly show that the aligned aspartic acid residue is critical for the catalytic activity of pseudouridine synthases from two additional families of these enzymes, supporting the predictive power of the sequence alignments and suggesting that the sequence motif containing the aligned aspartic acid residue might be a prerequisite for pseudouridine synthase function.

Role of cysteine residues in pseudouridine synthases of different families.

The pseudouridine synthases catalyze the isomerization of uridine to pseudouridine in RNA molecules. An attractive mechanism was proposed based on that of thymidylate synthase, in which the thiol(ate) group of a cysteine side chain serves as the nucleophile in a Michael addition to C6 of the isomerized uridine. Such a role for cysteine in the pseudouridine synthase TruA (also named Psi synthase I) has been discredited by site-directed mutagenesis, but sequence alignments have led to the conclusion that there are four distinct "families" of pseudouridine synthases that share no statistically significant global sequence similarity. It was, therefore, necessary to probe the role of cysteine residues in pseudouridine synthases of the families that do not include TruA. We examined the enzymes RluA and TruB, which are members of different families than TruA and each other. Substitution of cysteine for amino acids with nonnucleophilic side chains did not significantly alter the catalytic activity of either pseudouridine synthase. We conclude, therefore, that neither TruB nor RluA require thiol(ate) groups to effect catalysis, excluding their participation in a Michael addition to C6 of uridine, although not eliminating that mechanism (with an alternate nucleophile) from future consideration.