6月19日,，《自然》出版集團(tuán)旗下的Cell Research期刊在線發(fā)表了中國(guó)科學(xué)院生物物理研究所張榮光課題組在整合素重要激活因子Talin的自抑制機(jī)制方面的最新研究成果，論文題目為A novel membrane-dependent on/off switch mechanism of talin FERM domain at sites of cell adhesion,。
Talin自抑制復(fù)合物結(jié)構(gòu)圖
Talin自抑制狀態(tài)的解除模型
整合素是一種重要的細(xì)胞黏著分子,，在連接細(xì)胞內(nèi)外結(jié)構(gòu)的同時(shí),，還起著雙向信號(hào)轉(zhuǎn)導(dǎo)的作用。整合素胞外區(qū)具有兩種差異很大的構(gòu)像狀態(tài),，其與配體分子的結(jié)合力相差很遠(yuǎn),。而整合素胞外區(qū)的構(gòu)像狀態(tài)是由胞內(nèi)一系列激活因子的調(diào)控作用所決定的,。其中，Talin是最為重要的激活因子之一,。Talin蛋白本身也受到了嚴(yán)密的調(diào)控,，一般處于一種自抑制狀態(tài)：其N(xiāo)端的FERM結(jié)構(gòu)域會(huì)與C端的棒狀結(jié)構(gòu)域結(jié)合，遮擋了FERM結(jié)構(gòu)域上結(jié)合整合素胞內(nèi)區(qū)的位點(diǎn),。只有解除了自抑制狀態(tài),，Talin才能激活整合素。
在本研究中,，張榮光課題組與美國(guó)克里夫蘭醫(yī)學(xué)中心的秦軍教授課題組合作,，首先鑒定了參與Talin自抑制的準(zhǔn)確相互作用位點(diǎn)，構(gòu)建并分別表達(dá)了Talin蛋白N端和C端參與自抑制結(jié)合的子結(jié)構(gòu)域,，體外重組了Talin的自抑制復(fù)合物,，并進(jìn)而成功解析獲得了Talin自抑制復(fù)合物的晶體結(jié)構(gòu)。結(jié)構(gòu)比對(duì)發(fā)現(xiàn),，該復(fù)合物晶體結(jié)構(gòu)與之前報(bào)道的基于NMR數(shù)據(jù)的計(jì)算模型有相當(dāng)大的構(gòu)象差異,，兩結(jié)構(gòu)域之間的作用模式完全不同，呈現(xiàn)了90度的角度差異,。此外,，當(dāng)形成自抑制狀態(tài)時(shí)，兩結(jié)構(gòu)域朝向細(xì)胞膜一側(cè)的帶電性質(zhì)完全相反,，N端子結(jié)構(gòu)域?yàn)檎?，C端子結(jié)構(gòu)域?yàn)樨?fù)電。
基于對(duì)該晶體結(jié)構(gòu)的分析,，進(jìn)一步通過(guò)NMR手段對(duì)Talin突變體進(jìn)行了相互作用力的研究,，并測(cè)試了溶液中細(xì)胞膜磷脂分子頭部對(duì)Talin自抑制復(fù)合物的影響。結(jié)果顯示,，細(xì)胞膜負(fù)電表面的存在很可能有助于打開(kāi)Talin自抑制復(fù)合物,，從而幫助Talin解除自抑制狀態(tài)。據(jù)此,，本研究對(duì)于Talin自抑制狀態(tài)的解除提出了一個(gè)新的“推－拉”模型,，基于不同靜電力的相互作用，有別于經(jīng)典的“立體沖突”模型,。
本研究成果有助于深入理解Talin的調(diào)控機(jī)制,，乃至其對(duì)整合素的激活作用。（生物谷Bioon.com）
doi:10.1038/cr.2012.97
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A novel membrane-dependent on/off switch mechanism of talin FERM domain at sites of cell adhesion
Xianqiang Song,， Jun Yang,， Jamila Hirbawi， Sheng Ye,， H Dhanuja Perera,， Esen Goksoy,， Pallavi Dwivedi， Edward F Plow,， Rongguang Zhang,， Jun Qin
The activation of heterodimeric (α/β) integrin transmembrane receptors by cytosolic protein talin is crucial for regulating diverse cell-adhesion-dependent processes， including blood coagulation,， tissue remodeling,， and cancer metastasis. This process is triggered by the coincident binding of N-terminal FERM (four-point-one-protein/ezrin/radixin/moesin) domain of talin (talin-FERM) to the inner membrane surface and integrin β cytoplasmic tail， but how these binding events are spatiotemporally regulated remains obscure. Here we report the crystal structure of a dormant talin,， revealing how a C-terminal talin rod segment (talin-RS) self-masks a key integrin-binding site on talin-FERM via a large interface. Unexpectedly,， the structure also reveals a distinct negatively charged surface on talin-RS that electrostatically hinders the talin-FERM binding to the membrane. Such a dual inhibitory topology for talin is consistent with the biochemical and functional data， but differs significantly from a previous model. We show that upon enrichment with phosphotidylinositol-4,，5-bisphosphate (PIP2) – a known talin activator,， membrane strongly attracts a positively charged surface on talin-FERM and simultaneously repels the negatively charged surface on talin-RS. Such an electrostatic “pull-push” process promotes the relief of the dual inhibition of talin-FERM， which differs from the classic “steric clash” model for conventional PIP2-induced FERM domain activation. These data therefore unravel a new type of membrane-dependent FERM domain regulation and illustrate how it mediates the talin on/off switches to regulate integrin transmembrane signaling and cell adhesion.
