ROLE OF CELL SURFACE IN INITIATION OF CELL DIVISION

我们对凝血酶引发细胞分裂的机制进行了研究， 培养的成纤维细胞已经表明，在细胞表面的作用是 足以激活细胞。 这促使了对细胞表面的研究 刺激细胞分裂所必需的相互作用和事件。 这些研究导致了蛋白酶连接蛋白的鉴定， 介导凝血酶与细胞特异性结合的蛋白质 面 蛋白酶连接蛋白由细胞释放到培养基中， 它与凝血酶形成共价键。 凝血酶-蛋白酶连接蛋白 然后复合物特异性地结合细胞并被内化， 退化了 我们的研究表明，凝血酶与蛋白酶的连接 连接蛋白使凝血酶失活。 此外，蛋白酶连接蛋白抑制 凝血酶刺激，因此它代表了一种细胞 调节它们对凝血酶的促有丝分裂反应。 我们还展示了一个 未连接凝血酶的细胞表面结合位点， 凝血酶对该部位的刺激不是细胞生长所必需的。 师. 过去的研究表明， 凝血酶是细胞活化所必需的。 事实上，我们所有的研究结果 表明需要通过以下途径切割一种或多种细胞表面蛋白 凝血酶刺激。在这一点上，我们已经表明，几个细胞 表面蛋白被凝血酶切割。 其中之一已被确定 如纤连蛋白。 此外，约140种细胞表面蛋白 千道尔顿和55千道尔顿是凝血酶敏感的，但他们没有 尚未被确认。研究正在进行中，以更好地解决膜 这样我们就能够识别出其他可能 凝血酶敏感 我们将研究这些分裂中哪些是必要的 通过确定它们是否发生在一个大的系列中， 克隆的细胞群体对所述抗体应答或不应答， 凝血酶的促有丝分裂作用。 （一）

Our studies on the mechanism by which thrombin initiates division of cultured fibroblasts have shown that action at the cell surface is sufficient for cell activation. This prompted studies on the cell surface for interactions and events necessary for the stimulation of cell division. These studies led to the identification of protease nexin, a cell-secreted protein which mediates much of the specific binding of thrombin to the cell surface. Protease nexin is released by cells into the culture medium where it forms a covalent linkage with thrombin. The thrombin-protease nexin complexes then specifically bind to cells and are internalized and degraded. Our studies have shown that linkage of thrombin to protease nexin inactivates the thrombin. Moreover, protease nexin inhibits the stimulation by thrombin, and thus it represents a mechanism by which cells modulate their mitogenic response to thrombin. We have also demonstrated a cell surface binding site for unlinked thrombin and have shown that binding of thrombin to this site is not necessary for the stimulation of cell division. Past studies have shown that the proteolytic activity of thrombin is necessary for cell activation. In fact, all of our results point to a requirement for cleavage of one or more cell surface proteins by thrombin for the stimulation. At\this point we have shown that several cell surface proteins are cleaved by thrombin. One of these has been identified as fibronectin. In addition, cell surface proteins of about 140 kilodaltons and 55 kilodaltons were thrombin-sensitive, but they have not yet been identified. Studies\are underway to better resolve membrane proteins so we will be able to identify additional ones that might be thrombin-sensitive. We will study which of these cleavages are necessary for cell activation by determining whether they occur in a large series of cloned cell populations that are either responsive or unresponsive to the mitogenic action of thrombin. (A)