Aortic endothelial cell proteoheparan sulfate. I. Isolation and characterization of plasmamembrane-associated and extracellular species.
Proteoheparan sulfate biosynthesis was studied in cultured bovine aortic endothelial cells by means of pulse and pulse-chase experiments and subcellular fractionations. Three proteoheparan sulfate species were found in the medium. The major species, which the authors have called HS I, appeared in the medium only after an initial lag period and was also found associated with the plasma membrane. The other two (HS II and HS III) appeared in small amounts in the medium at early time points. At later times these were not readily observed because the large amounts of HS I present in the medium. The major medium species, HS I, appeared to be composed of approximately four heparan sulfate chains of approximately 35,000 daltons and a core protein of approximately 55,000 daltons apparent molecular weight. HS I appeared to be homogeneous by gel filtration on Sepharose CL 2B and 6B and elution from DEAE Sephacel, electrophoresis on Nu-Sieve agarose, and CsCl density centrifugation. After digestion with heparinase the core protein appeared to be homogeneous by S-200 Sephacel chromatography. HS I was also found associated with plasma membrane fractions of the cultured bovine aortic endothelial cells, and antisera raised against it stained epithelial and endothelial cells in patterns consistent with a cell surface localization. Of the other two species found in the medium, one (HS II) also appeared to be a component of the cell layer. This species appeared to contain approximately four heparan sulfate chains of approximately 20,000 daltons apparent molecular weight. Antisera raised against a similar molecule produced by HR 9 cell cultures stained basement membranes intensely, supporting the subcellular matrix localization of this molecule. The third species (HS III) was detected in culture medium only and apparently contained two heparan sulfate chains of approximately 20,000 daltons apparent molecular weight. These results support the concept of multiple endothelial cell proteoheparan sulfate species which exhibit differences in structure and localization and possibly diverse specialized functions.