EXPRESSION OF CARBOHYDRATE RESIDUES IN PLASMA-MEMBRANE GLYCOPROTEINS DURING THE DIFFERENTIATION OF AMPHIBIAN EPIDERMAL-CELLS

Expression of various sugar residues on the plasma membrane of frog (Rana per perezi) epidermal cells at different stages of differentiation has been monitored with the use of a battery of HRP-conjugated lectins. In paraffin-embedded tissue, mannose residues (stained by Concanavalin A) were detected at the keratinocyte cell surface in all epidermal strata. However, Lens culinaris is agglutinin (LCA), also specific for mannose, specifically stained the plasma membrane of cells from the stratum germinativum. Expression of N-acetyl-glucosamine (GlcNAc), labelled with wheat germ agglutinin (WGA), was maximum at the cell surface of basal cells and progressively decreased through the stratum spinosum. Galactose (Gal) and N-acetyl-galactosamine (GalNAc) residues, labelled with Griffonia simplicifolia I (GS I) and Glycine max (SBA) agglutinins, respectively, were expressed according to the degree of differentiation in amphibian epidermal cells. Sialic acid-containing glycoproteins, labelled with Limax flavus agglutinin (LFA), were found in the outermost plasma membrane of the replacement cell layer and stratum corneum. Glycoproteins responsible for the observed lectin-binding patterns have been identified by staining on nitrocellulose filters after electrophoresis of solubilized plasma membrane fractions and Western blotting. Changes at the level of glycosylation of plasma membrane glycoproteins as epidermal cells differentiate are discussed on the basis of a progressive addition of Gal residues. Integral membrane proteins have been solubilized with the non-denaturing detergent CHAPS and glycoproteins containing terminal Gal residues, that are expressed according to the degree of differentiation in frog epidermis, have been partially purified by affinity chromatography on a GSI-Sepharose 4 B column. The purified fraction was composed by four acidic glycoproteins with isoelectric points between 4.6 and 5.2 and, in SDS-gels gave five major protein bands with approximate molecular weights of 148, 140, 102, 60, and 52 kDa in SDS-gels. The 102 and 52 kDa bands correspond to the alpha and beta subunits of amphibian epidermal Na+,K+-ATPase as demonstrated by specific staining with a polyclonal antibody against the catalytic subunit of pig kidney proton pump and staining with lectins GSI, GSII, and WGA. Possible relationships between higher molecular weight proteins and the constituents of intramembranous particles from the outermost plasma membranes of the replacement cell layer and the stratum corneum are also discussed.