Isolation and partial characterization of a cystine-rich basic heparin-binding protein from bovine platelets

We report the isolation and partial characterization of a so far unidentified basic platelet protein. Delipidated bovine platelets were extracted at pH 2.1. The extract was subjected to differential precipitation at pH 5.4-5.5 and by ammonium sulfate, then it was further purified by ion exchange chromatography on DEAE and CM cellulose columns in an urea containing medium. The major protein peak eluted from the CM cellulose column by NaCl gradient contained a protein in electrophoretically homogeneous form. It consists of a single polypeptide chain with an Mr of 28,000 as estimated by SDS PAGE. It was shown to be extremely rich in lysine and cystine and possessed a highly basic character (pI 9.8-10.1). On this basis the term cystine-rich basic protein (CRBP) was proposed for the new protein. Unlike some other low Mr basic proteins it did not bind calmodulin and troponin C, however, it showed significant heparin neutralizing activity.