Structure-function studies of a colony stimulating factor (CSF-1)

CSF-1 is a glycoprotein growth factor which specifically stimulates the survival, proliferation, and differentiation of cells of the mononuclear phagocytic lineage. In this study, microgram amounts of radiolabeled murine L-cell and human urinary CSF-1 were isolated in pure form and used to investigate the nature and extent of CSF-1 glycosylation and the requirment of the carbohydrate moiety for its biological and antibody-binding activities. The molecular weight of the preparations examined varied betwen ~ 47,000 and ~ 76,000. Reduction and alkylation halved the molecular weight of all preparations, reflecting within each the existence of two similar subunits. Exhaustive treatment of reduced and alkylated CSF-1 with endo-β-N-acetylglucosaminidase D, but not endo-β-N-acetylglucosamindase H, gave rise to a molecular of M(r) ~ 16,500 of which the polypeptide portion accounted for ~ 14,500. Whereas the parent molecule bound concanavalin A, this product did not, indicating that heterogeneity in the saccharide component could explain the observed variation in the molecular weight of CSF-1. These results also suggest that the two polypeptide chains in the dimeric CSF-1 molecule are very similar and posibly even identical. Because of the specificity of endo-β-N-acetylglucosaminidase D, it is concluded that the carbohydrate moieties are Asn-lined 'complex-type' units. Examination of the effects of endo-α-N-acetyl-D-galactosaminidase treatment on the molecular weight of reduced and alkylated CSF-2 failed to provide evidence for the existence of O-glycosidically linked oligosaccharides. Treatment of native CSF-1 with endo-β-N-acetylglucosaminidase D was almost as efficient in removing carbohydrate as in the case of the reduced and alkylated subunits. Removal did not cause loss of antibody binding, receptor binding, or biological activity.