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

Abstract

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 requirement of the carbohydrate moiety for its biological and antibody-binding activities. The molecular weight of the preparations examined varied between approximately 47,000 and approximately 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-beta-N-acetylglucosaminidase D, but not endo-beta-N-acetylglucosaminidase H, gave rise to a molecule of Mr approximately 16,500 of which the polypeptide portion accounted for approximately 15,500. Whereas the parent molecule bound concanavalin A, the 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 possibly even identical. Because of the specificity of endo-beta-N-acetylglucosaminidase D, it is concluded that the carbohydrate moieties are Asn-linked "complex-type" units. Examination of the effects of endo-alpha-N-acetyl-D-galactosaminidase treatment on the molecular weight of reduced and alkylated CSF-1 failed to provide evidence for the existence of O-glycosidically linked oligosaccharides. Treatment of native CSF-1 with endo-beta-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.