Deletion of carboxy-terminal residues of murine granulocyte-macrophage colony-stimulating factor results in a loss of biologic activity and altered glycosylation

Evaluation of human N-linked glycosylation sites in murine granulocyte-macrophage colony-stimulating factor

Nonglycosylated murine and human granulocyte-macrophage colony-stimulating factor have a molecular mass of approximately 14.5 kDa predicted from the primary amino acid sequence. The expression of both proteins in COS cells leads to a heterogeneous population of molecules that differ in the degree of glycosylation. Both human and murine molecules contain two N-linked glycosylation sites that are situated in nonhomologous locations along the linear sequence. Despite this difference both proteins show a similar size distribution among the glycosylation variants. These studies analyze the effects of introducing in the murine protein novel N-linked glycosylation sites corresponding to those sites found in the human molecule. A panel of molecules composed of various combinations of human N-linked glycosylation sites in either the presence or the absence of murine N-linked glycosylation was compared. Substitution of a proper human N-linked glycosylation consensus sequence at Asn 24 did not result in N-linked glycosylation, nor was there any considerable effect on bioactivity. Replacement of the N-linked glycosylation consensus sequence at Asn 34 results in glycosylation similar to that found in the human molecule and causes a significant decrease in bioactivity. These data suggest that the position of N-linked glycosylation is critical for maximal bioactivity in a particular species and that the changes in position of these sites in different species probably occurred during evolution in response to changes in their receptors.

Mammalian cell gene expression: protein glycosylation

Considerable advances have been made in identifying the factors determining the glycosylation pattern of glycoproteins secreted by mammalian cells. This has allowed a greater appreciation of the way in which recombinant proteins may be glycosylated after expression in a heterologous system. The studies reviewed herein extend the wider view that glycosylation of native and recombinant proteins is a complex event dependent on the protein moiety, the host cell, and also the environment in which transfected cells are cultured. The details of the way in which these factors combine to establish the glycosylation pattern of a secreted protein are now beginning to be unravelled.

A neutralizing monoclonal antibody binds to an epitope near the amino terminus of murine granulocyte-macrophage colony-stimulating factor

A rat anti-murine granulocyte-macrophage colony-stimulating factor (mGM-CSF) monoclonal antibody, A2, that neutralizes bioactivity in vitro was isolated. The binding epitope recognized by this antibody was identified using human-murine hybrid GM-CSF proteins. A2 was unable to immunoprecipitate a hybrid (hm7) protein containing the human GM-CSF sequence for the first 11 amino terminal amino acids, and the mGM-CSF sequence for amino acids 12-124. In contrast, A2 did recognize a hybrid which substitutes human GM-CSF amino acids 23-36 in the murine sequence. These data suggest that this neutralizing antibody recognizes an epitope at the amino terminus of mGM-CSF. Because hm7 did maintain in vitro bioactivity, it is probable that the epitope recognized by the neutralizing antibody is not itself part of the receptor-binding domain of mGM-CSF; rather, it is likely that neutralization occurs as a result of antibody binding near the receptor-binding site, with steric inhibition of mGM-CSF binding to its receptor. Interestingly, monoclonal antibody A2 does not recognize mGM-CSF glycosylation species corresponding to predicted maximal O-glycosylation variants. The presence of O-glycosylation sites within the antibody-binding epitope was confirmed using site-directed mutagenesis. Potential O-glycosylation sites in native mGM-CSF were removed by introducing conservative amino acid substitutions, and expected molecular weight reductions were obtained. These findings are consistent with previous reports that suggest the importance of the integrity of residues near the amino terminus to GM-CSF bioactivity.