Sulfate contributes to the negative charge of podocalyxin, the major sialoglycoprotein of the glomerular filtration slits

Abstract

Podocalyxin is the major sialoprotein of the rat glomerulus. Its function is to maintain the filtration slits of the glomerular epithelium open by virtue of its high net negative charge. We have used biosynthetic labeling and oligosaccharide analysis to characterize the anionic-charge-carrying moieties on this protein. Kidney slices from 2-day-old rats were biosynthetically labeled with [35S]Cys, [3H]Man, [3H]GlcN, and 35SO4, after which podocalyxin was immunoprecipitated and purified by SDS/PAGE. All these labels were incorporated into podocalyxin. Immunoprecipitates were subjected to digestion with specific glycosidases or digested with Pronase followed by chromatographic analysis of the released glycopeptides. Podocalyxin was susceptible to digestion with N-Glycanase and O-Glycanase, indicating the presence of both N- and O-linked oligosaccharides. Approximately 30% of the [3H]GlcN-labeled glycopeptides bound to Con A, confirming the presence of high mannose, hybrid, or biantennary N-linked structures; alkaline borohydride treatment confirmed the presence of O-linked oligosaccharides. Analysis of the 35SO4-labeled glycopeptides indicated that both the N- and O-linked structures were sulfated. We conclude that in newborn rat kidney (i) podocalyxin contains both O- and N-linked oligosaccharides [high mannose or hybrid type, biantennary, and complex (sialylated) type], (ii) podocalyxin is sulfated, and (iii) sulfate is located on both O-linked oligosaccharides and on glycopeptides carrying tri- or tetrantennary N-linked structures. These results indicate that the net negative charge of podocalyxin is most likely derived from sulfate as well as from sialic acid residues.