The structure of endothelial cell thrombospondin

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Abstract
The glycoprotein thrombospondin is distributed between the extracellular matrix and the platelet‐sequestered pool in the resting state and it undergoes redistribution upon platelet stimulation. It is believed to play a role in matrix structure and in coagulation. We have studied the structural domains of endothelial cell (EC) thrombospondin by use of the serine proteases thrombin, trypsin and chymotrypsin and have characterized the heparin‐binding domains of this molecule. For this purpose we used purified thrombospondin synthesized and secreted by bovine aortic endothelial cells grown in the presence of radiolabeled methionine. We find that the susceptibility of EC thrombospondin to proteolysis is five‐fold smaller than that of platelet thrombospondin. In the presence of 2 mM Ca ions the molecule is cleaved by 20 U/ml thrombin at a single locus, to yield fragments of 160 kDa and 35 kDa. Trypsin digestion for 5 min at room temperature at an enzyme‐to‐substrate ratio of 1:20 produces a stable fragment of 140 kDa but not the 30‐kDa fragment observed in platelet thrombospondin. Chymotrypsin, under identical conditions to those used for trypsin, cleaves EC thrombospondin into four stable fragments of 160 kDa, 140 kDa, 27 kDa and 18 kDa. Chelation of Ca by EDTA increases susceptibility of the molecule to proteolysis. Under the conditions used a cryptic thrombin‐cleavage site, not hitherto observed in platelet thrombospondin, was observed in EC thrombospondin. The location of this site is near a chymotrypsin‐susceptible site, which has been observed in the long connecting arm, which is particularly Ca‐stabilized. Heparinbinding capacity of EC thrombospondin was observed in at least two separate loci. Both thrombin and chymotrypsin produced small fragments (35 kDa and 27 kDa respectively) which bound to heparin with high affinity, and large fragments (160 kDa for thrombin and 140 kDa for chymotrypsin) which had low affinity. Chelation of Ca substantially decreased the low‐affinity binding of the large fragments but not the high‐affinity binding of the small fragments. Two‐dimensional gel electrophoresis of the chymotryptic heparin‐binding fragments shows that each molecule gave rise to a heterogeneous array of fragments of high molecular mass bound by disulfide bonds, indicating that there is a difference in the rate of cleavage between the three subunits of EC thrombospondin. Trypsin, despite its limited degradation, completely eliminated the heparin‐binding capacity of both high and low‐affinity loci, in contrast to platelet thrombospondin where the high affinity remains intact. Our data suggest that EC thrombospondin is less susceptible to degradation and removal than platelet thrombospondin and could, therefore, function under harsher conditions as perhaps exist at the platelet – matrix interface following activation of coagulation.