Human tissue factor was phosphorylated when incubated with lysates of U87-MG cells or fractions from preparative isoelectric focusing of the lysates. The cytoplasmic domain peptide, isolated following chemical cleavage at cysteine 245, focused on PhastGel IEF near pH 3.4, indicating the presence of three phosphate groups. A peptide corresponding to the carboxyl-terminal cytoplasmic domain (residues 245-263) was synthesized and shown to be a protein kinase substrate when incubated with lysates of U87-MG cells and radiolabeled ATP. As found with full-length tissue factor, the TF245- 263 peptide was phosphorylated at all three serines, but a diphosphate form was also identified. TF245-263 was phosphorylated in the absence of calcium as well as in the presence of calphostin C, indicating that phosphorylation can be independent of protein kinase C. These results reveal that tissue factor can be multiply phosphorylated in vitro, and that the synthetic TF245-263 cytoplasmic domain peptide serves as a model substrate.
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