Structure and activity of recombinant human interferon-gamma analogs

Construction and analysis of a profile library characterizing groups of structurally known proteins

A new sequence motif library StrProf was constructed characterizing the groups of related proteins in the PDB three-dimensional structure database. For a representative member of each protein family, which was identified by cross-referencing the PDB with the PIR superfamily classification, a group of related sequences was collected by the BLAST search against the nonredundant protein sequence database. For every group, the motifs were identified automatically according to the criteria of conservation and uniqueness of pentapeptide patterns and with a dual dynamic programming algorithm. In the StrProf library, motifs are represented by profile matrices rather than consensus patterns to allow more flexible search capabilities. Another dynamic programming algorithm was then developed to search this motif library. When the computationally derived StrProf was compared with PROSITE, which is a manually derived motif library in the best consensus pattern representation, the numbers of identified patterns were comparable. StrProf missed about one third of the PROSITE motifs, but there were also new motifs lacking in PROSITE. The new library was incorporated in SMART (Sequence Motif Analysis and Retrieval Tool), a computer tool designed to help search and annotate biologically important sites in an unknown protein sequence. The client program is available free of charge through the Internet.

Structural elements required for receptor recognition of human interferon-gamma

Interferon (IFN)-gamma is a central factor in numerous immune responses. Recently the three-dimensional structure of human and rabbit IFN-gamma has been elucidated. This review attempts to bring together the structure and function information into a working model of IFN-gamma: receptor interaction. Based on mutagenesis studies, and corroborated by work with peptides, antibodies and proteolytic digestion, three regions have been found to be important for receptor binding: a long loop connecting the A and B helices, His111 in the F helix and a conserved section of the flexible carboxyl terminus. These three regions may form one continuous binding domain.

Mutations in human interferon gamma affecting inclusion body formation identified by a general immunochemical screen

High level expression of the gene for human interferon-gamma (HuIFN-gamma) in E. coli JM101 cultured at 37 degrees C results in the distribution of over 90 percent of the total accumulated gene product into inclusion bodies (IBs). We have identified mutations throughout the molecule that alter the distribution between the soluble and inclusion body fractions without greatly affecting total expression level. Some mutants retain high biological activity but are localized almost entirely in the soluble fraction. Mutations affecting IB distribution as well as stability to intracellular proteolysis were detected by immunochemical screens and verified by gel assays. Immunochemical screens such as those employed here may allow identification of folding and stability mutants in heterologously expressed proteins when there is no other basis for selection or screening. These results also suggest that one solution to production problems arising from IB formation may be to identify mutations in the target protein that favor expression of soluble protein while retaining biological activity.

Interferon-gamma can remain on the cell surface during the induction of the antiviral state

The antiviral activity of cell-associated, non-elutable recombinant human gamma interferon (rHuIFN-gamma) was neutralized by antibody. The neutralization of cell-associated rHuIFN-gamma was maximal through 2 h (60-100%) and declined through 8 h (20-40%). Concomitantly, the antiviral activity of cell-associated [Met-Gln-Asp-Pro]-rHuIFN-gamma was sensitive to trypsin digestion over the same time period. However, the cell-associated antiviral activity of [Cys-Tyr-Cys]-rHuIFN-gamma remained sensitive to trypsin through 8 h. Neutralization of cell-associated rHuIFN-gamma by antibodies to the N-terminal end of HuIFN-gamma suggests that the N-terminal end(s) of cell-associated rHuIFN-gamma is directed outward from the receptor. Further, immunoprecipitation of radio-labelled rHuIFN-gamma by antibody alone suggests that biologically active rHuIFN-gamma is an oligomer. Taken together, these studies suggest that neutralization of cell-associated rHuIFN-gamma is probably due to divalent binding of antibody to or between rHuIFN-gamma in receptors on the cell surface. Also, our studies indicate that rHuIFN-gamma can remain associated with the cell surface during the induction of the antiviral state (AVS) and that binding of antibody to cell-associated rHuIFN-gamma inhibits the molecular events responsible for induction of the AVS.

Effect of amino-terminal processing by Staphylococcus aureus V-8 protease on activity and structure of recombinant human interferon-gamma

Treatment of recombinant human interferon-gamma (rHuIFN-gamma) with Staphylococcus aureus V-8 protease generated a transiently stable species that lacks 10 amino-terminal residues. This protein showed distinct secondary and higher-order structures with an alpha-helical content of 31%, suggesting that the secondary and tertiary structure largely remain upon removal of 10 amino-terminal residues. The antiviral activity was abolished, or greatly reduced, for this species relative to the intact protein. These results suggest an important role for the amino-terminal portion in the activity of human interferon-gamma. Since the digested protein is difficult to refold from the acid-denatured state, it was concluded that, although not essential for maintaining the core structure of the protein, the amino-terminal portion is critical for refolding the protein from acid.

Epitope mapping of recombinant human gamma interferon using monoclonal antibodies

Five monoclonal antibodies (MAbs B22, B27, 3-6, 32 and 35) specific for human recombinant IFN-gamma were characterized. These MAbs were used to set up quantitative sandwich ELISAs which allowed the detection of 1.25 ng/ml of IFN-gamma when diluted in normal human serum. Epitope mapping of the IFN-gamma molecule using these MAbs demonstrated that antibodies 3-6 and 32 which did not inhibit the biological activity of IFN-gamma recognized an epitope localized on the 15 C-terminal amino acids, suggesting that this portion of the molecule was not implicated in the biological activity of IFN-gamma. Sandwich ELISAs were performed using various pairs of MAbs. The level of reactivity obtained when antibodies B22 and B27 were used simultaneously as catcher and tracer was similar to the result obtained with two antibodies recognizing different epitopes. These results confirm that the IFN-gamma molecule is a dimer in solution and indicate that the two sites of the IFN-gamma dimeric molecule which are associated with the biological activity (epitope B22/B27) are fully exposed. In contrast, the C-terminus is only partially accessible to the antibodies 3-6/32, suggesting that the dimerization of IFN-gamma molecule results in the interaction of regions of the monomers that are homologous and adjacent to the C-terminus.

Alterations of the amino terminus of murine interferon-gamma: expression and biological activity

To examine the relationship of murine interferon-gamma (MuIFN-gamma) protein structure to its biological function, several analogs (amino-terminal deletions) of MuIFN-gamma (type II IFN) have been constructed using synthetic oligonucleotide-directed primer repair. High-level expression was achieved through evaluation of such parameters as plasmid origin of replication, antibiotic resistance markers, and host background. The antiviral activity of these analogs varied depending upon the number of amino acids deleted. When the first nine amino acids of IFN-gamma were eliminated (10-136 analog), the protein product lacked detectable antiviral activity. The 10-136 analog but not the 12-136 analog was detectable by Western blots or Coomassie staining of protein gels. These results present the high-level (optimized) expression of several analogs of MuIFN-gamma and further define the role of the amino terminus of the protein with respect to antiviral activity.

Reduction in antiviral activity of human interferon-gamma in acidic media with reference to structural change

The fluorescence intensity of a unique tryptophan 36 in human interferon-gamma was drastically decreased below pH 4 with a concomitant decrease of antiviral activity. The region of residues 32-42 of human interferon-gamma was found by calculation to have a low hydrophobicity together with a high helical hydrophobic moment, and the net electric charge of this region having an amphiphilic helical structure changed significantly near pH 4. These results suggest that the region of residues 32-42 plays an important role in exhibiting antiviral activity.