Affinity labeling of high-affinity alpha-thrombin binding sites on the surface of hamster fibroblasts

The serine proteinase alpha-thrombin potently stimulates reinitiation of DNA synthesis in quiescent Chinese hamster fibroblasts (CCL39 line). 125I-labeled alpha-thrombin binds rapidly and specifically to CCL39 cells with high affinity (Kd approximately 4 nM). Binding at 37 degrees C was found to remain stable for 6 h or more during which time no receptor down-regulation, ligand internalization and/or degradation could be detected. The structure of alpha-thrombin receptors on CCL39 cells was identified by covalently coupling 125I-alpha-thrombin to intact cells using a homobifunctional cross-linking agent, ethylene glycol bis(succinimidyl succinate). By resolution in sodium dodecyl sulfate polyacrylamide gel electrophoresis we observed the specific labeling of a major alpha-thrombin-binding site of Mr approximately 150 000 revealed as a 125I-alpha-thrombin cross-linked complex of Mr approximately 180 000. Independent of chemical cross-linking, 125I-alpha-thrombin also formed a covalent complex with a minor, 35 000 Mr, membrane component identified as protease nexin. Two derivatives of alpha-thrombin modified at the active site are 1000-fold less than alpha-thrombin for mitogenicity. These two non-mitogenic derivatives bound to cells with similar affinity and maximal binding capacity as native alpha-thrombin, and affinity-labeled the receptor subunit of Mr 150 000. When present in large excess, during incubation of cells with alpha-thrombin, these binding antagonists were ineffective in blocking alpha-thrombin-induced DNA synthesis. These data suggest that the specific 150 000 Mr binding sites that display high affinity for alpha-thrombin do not mediate induction of the cellular mitogenic response.