N-glycosylation of human interferon-gamma: glycans at Asn-25 are critical for protease resistance.

Human interferon-gamma (IFN-gamma) is a secretory, dimeric glycoprotein that forms a compact globular structure with potential N-linked glycosylation sites at Asn-25 and Asn-97 on the surface of the dimer. In natural leucocyte IFN-gamma (nIFN-gamma), 52%, 39% and 9% of the monomers are core-glycosylated in two, one or none of the potential N-glycosylation sites respectively. Chemical cross-linking of nIFN-gamma with glutaraldehyde revealed that 4, 3, 2 or 1 glycosylation sites occupied 28%, 40%, 26% and 6% of the dimers respectively. In baculovirus-produced wild-type (Wt) and N-linked glycosylation site-defective mutant (N25Q or N97Q, Asn-25 or Asn-97 substituted by Gln) IFN-gamma proteins, the extent of core glycosylation of monomers reflected the glycan composition of dimers. This suggests that dimers are formed randomly and independently of glycosylation. The glycan residues of IFN-gamma, especially at Asn-25, play an important role in protease resistance. Unglycosylated recombinant IFN-gamma proteins (from Escherichia coli and baculovirus) and N25Q IFN-gamma were sensitive to crude granulocyte protease, purified elastase, cathepsin G and plasmin degradation. Fully glycosylated nIFN-gamma and baculovirus Wt and N97Q IFN-gamma showed full or partial resistance to these proteases. These results emphasize the importance of glycan residues, especially at Asn-25, in the proteolytic stability of human IFN-gamma. Whether the differential glycosylation of n- and recombinant IFN-gamma (rIFN-gamma) is reflected in their biological activities in tissues or their clinical applicability is not known.