Location and partial characterization of the heparin-binding fragment of platelet thrombospondin

Purified platelet thrombospondin (TS) was subjected to proteolysis with a number of proteases including factors IXa, Xa, thrombin, elastase, trypsin, and chymotrypsin. All enzymes yielded fragments of TS which bound to heparin-Sepharose. Only chymotrypsin cleavage produced a single species of heparin-binding fragment, as analyzed by SDS-PAGE. This fragment had a chain molecular weight of 28,000, and contained no interchain disulfide bonds. Amino acid sequence analysis of the heparin-binding fragment and of TS revealed a single sequence, indicating that the fragment constitutes the amino-terminal domain of TS and that the three chains in TS are identical in this region.