The cell specificity and biosynthesis of mouse glycophorins studied with monoclonal antibodies

Murine erythropoiesis represents a favourable system in which to investigate the coordinate regulation of gene expression due to the availability of erythroid precursor cells at various stages of differentiation. In this report, we investigate the biosynthesis and cell specificity of two characteristic murine RBC membrane glycoproteins that resemble the human RBC glycophorins: a major component of apparent molecular mass 31 kD (glycophorin MA) and a minor 46 kD component (glycophorin MB). Both glycophorins bind to wheat germ lectin and share a common protein antigenic determinant recognised by a monoclonal antibody (GP 29.4), but they differ significantly in their carbohydrate components: whilst both glycophorins contain mainly O-linked sugars, glycophorin MA contains in addition at least one N-linked carbohydrate residue and terminal sialic acid residues. Pulse-chase in vivo labelling experiments combined with in vitro translations of glycophorin mRNAs show that the initial precursor to glycophorin MA is a 24.5 kD polypeptide which is subsequently processed and glycosylated to give the mature 31 kD molecule via a 21.5 kD polypeptide intermediate. Both glycophorins MA and MB are synthesized most actively in early to mid erythroblasts (e.g., Friend cells induced for 3 days with DMSO) but their synthesis is considerably reduced by the reticulocyte stage. However, of the other cell types tested (neuroblastoma, myeloma, fibroblasts, epithelial cells and T-lymphoma cells), none synthesizes glycophorin with the possible exception of a low level in thymus tissue. Thus murine glycophorins, in contrast to the RBC cytoskeletal proteins (spectrin, ankyrin, band 4.1) seem to be restricted to the erythroid cell lineage like human glycophorin.