探討人類似Ste20激脢，MST3，所誘發的細胞凋亡

Abstract

MST3是人類絲胺酸/蘇胺酸激脢的一員，相似於出芽酵母菌的激脢STE20，隸屬於哺乳類似SPS1/STE20激脢家族。MST3在人類細胞中確切的功能及生理調控機制尚未可知，但似乎在誘發細胞凋亡上扮演角色。經由對MST3的蛋白質序列的研究，我們可推斷出MST3包含兩種特定訊息，一者為在半光胺酸切位點之前的一段細胞核內移序列（nuclear localization signal）；另一則為位在羰端的細胞核外移序列（nuclear export signal）。先前的報告顯示，同時具有細胞和內移跟外移序列的蛋白質可自胞內不同區域穿梭。初步的結果指出MST3全長的野生株主要存在於細胞質中，而只包含MST3激脢部位的截斷株則大多位在於細胞核內。將細胞中大量表現MST3並施以一CRM1/exprotein阻斷劑Leptomycin B的處理，可使MST3野生株迅速失去其原來在細胞質中局部存在的特性。此外，若在細胞中大量表現已截斷細胞核外移序列的MST3的突變株亦可得到相似的結果。這些現象指出MST3野生株在細胞質中局部存在的特性是經由其羰端的細胞核外移序列，藉由CRM1/exprotin所媒介的主動運輸所調控。此外，經由CD95/Fas 接和作用的刺激以及TNF-α和Staurosporine的處理可致使暫時表現MST3的HeLa細胞株發生嚴重的細胞死亡，並發現MST3有細胞質轉移的現象。根據這些結果，我們推測MST3是在細胞中的功能是為了回應特定的細胞壓力而促成細胞凋亡。但關於MST3所誘發的細胞凋亡跟胞內轉移的性質間的關連仍須進一步研究。

MST3, a member of human ste20-like serine/threonine kinase belongs to a mammalian SPS1/STE20-like kinase family. Although the exact cellular function and physiological regulation of MST3 in human cells is not yet known but it is likely that it plays a role in promoting apoptosis. We found that MST3 contains two putative regions, one is nuclear localization signal (NLS) located prior to the caspase cleavage site (D313) and the other is a leucine-rich carboxyl terminal nuclear export signal (NES). Preliminary results indicated that wild type of full length MST3 was present predominantly in the cytoplasm, and the truncated-form MST3, that only contains the kinase domain, was mainly pleasant in the nucleus. Overexpressing of wild type MST3 treated with leptomycin B, a CRM1/exportin inhibitor, lost cytoplasmic localization. Furthermore, overexpression of a truncated mutant lacking C-terminal NES regulatory domain also showed the same result. These phenomenons indicated that cytoplasmic localization of MST3 regulated by a C-terminal regulatory region through CRM1/exportin-mediated active transport. Induction of stimuli, such as CD95/Fas ligation, TNF-α, and staurosporine causes MST3-expressed HeLa cells apoptosis. In a similar experiment, nuclear localization of EGFP-labeled MST3 was observed in the HeLa cells. Accordingly, we suggest that the cellular function of MST3 is required for apoptosis in response to certain cellular stimuli. However, further studies are needed to elucidate the relationship between nuclear translocation and apoptosis caused by MST3.