Heparan sulfate proteoglycans from mouse mammary epithelial cells. A putative membrane proteoglycan associates quantitatively with lipid vesicles.

Mouse mammary epithelial (NMuMG) cells produce both cellular and extracellular heparan sulfate-rich proteoglycans. A cellular proteoglycan, but no extracellular proteoglycans, associates quantitatively and vectorially with lipid vesicles, as assessed by column chromatography and centrifugation. This lipophilic cellular proteoglycan is extracted as an aggregate when cells are treated with 4 M guanidine HCl, but is extracted as a single component in the presence of detergent, suggesting that it aggregates with cellular lipid. An association with lipid is confirmed by intercalation of the proteoglycan into the bilayer of lipid vesicles. Formation of lipid vesicles in the presence of the proteoglycan causes the proteoglycan to have the chromatographic and sedimentation behavior of the vesicles while destruction of the vesicles with detergent nullifies this effect. The proteoglycan is intercalated nullifies this effect. The proteoglycan is intercalated into the vesicles with its glycosaminoglycan-containing domain exposed to the exterior since mild trypsin treatment quantitatively removes this portion of the proteoglycan from the vesicle. After cleavage from the vesicle, the released proteoglycan chromatographs with an apparent molecular size similar to that of the whole proteoglycan, but no longer aggregates with lipid. Thus, trypsin removes a lipophilic domain which is responsible for its interaction with lipid and presumably anchors the proteoglycan in cellular membranes.