Functional analysis of Tec family kinases in normal hematopoiesis and hematological disease.

小鼠sak基因编码一种假定的丝氨酸-苏氨酸激酶，该激酶与Plk/Polo家族的成员同源。虽然Sak蛋白被认为参与细胞生长机制，但未能证实其激酶活性。因此，很少有人阐明Sak是如何调控的，以及Sak如何促进细胞增殖。Tec是一种细胞质蛋白酪氨酸激酶（PTK），在细胞因子受体、淋巴细胞表面抗原、异三聚体G蛋白连接受体和整合素的刺激下被激活。为了明确Tec在体内的功能，我们尝试用酵母双杂交筛选的方法分离Tec的第二信使。其中一种tec结合蛋白被证明是Sak。在人肾293细胞中，Sak被Tec酪氨酸磷酸化，且仅在Tec存在下检测到Sak的丝氨酸-苏氨酸激酶活性，提示Sak是Tec的效应分子。此外，Tec活性有效地保护Sak免受“PEST”序列依赖性蛋白水解。PEST序列的内部缺失导致Sak蛋白的稳定，这些突变体的表达抑制细胞生长。我们的数据共同支持了Sak在ptk介导的信号通路中的新作用。

The murine sak gene encodes a putative serine-threonine kinase which is homologous to the members of the Plk/Polo family. Although Sak protein is presumed to be involved in cell growth mechanism, efforts have failed to demonstrate its kinase activity. Little has been, therefore, elucidated how Sak is regulated and how Sak contributes to cell proliferation. Tec is a cytoplasmic protein-tyrosine kinase (PTK) which becomes activated by the stimulation of cytokine receptors, lymphocyte surface antigens, heterotrimeric G protein-linked receptors, and integrins. To clarify the in vivo function of Tec, we have tried to isolate the second messengers of Tec by using the yeast two-hybrid screening. One of such Tec-binding proteins turned out to be Sak. In human kidney 293 cells, Sak became tyrosine-phosphorylated by Tec, and the serine-threonine kinase activity of Sak was detected only under the presence of Tec, suggesting Sak to be an effector molecule of Tec. In addition, Tec activity efficiently protects Sak from the "PEST" sequence-dependent proteolysis. Internal deletion of the PEST sequences led to the stabilization of Sak proteins, and expression of these mutants acted suppressive to cell growth. Our data collectively supports a novel role of Sak acting in the PTK-mediated signaling pathway.