6月19日,，《自然》出版集團旗下的Cell Research期刊在線發(fā)表了中國科學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自抑制復合物結構圖
Talin自抑制狀態(tài)的解除模型
整合素是一種重要的細胞黏著分子,，在連接細胞內外結構的同時,，還起著雙向信號轉導的作用,。整合素胞外區(qū)具有兩種差異很大的構像狀態(tài),，其與配體分子的結合力相差很遠。而整合素胞外區(qū)的構像狀態(tài)是由胞內一系列激活因子的調控作用所決定的,。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密的調控,，一般處于一種自抑制狀態(tài)：其N端的FERM結構域會與C端的棒狀結構域結合,，遮擋了FERM結構域上結合整合素胞內區(qū)的位點。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克里夫蘭醫(yī)學中心的秦軍教授課題組合作,，首先鑒定了參與Talin自抑制的準確相互作用位點,，構建并分別表達了Talin蛋白N端和C端參與自抑制結合的子結構域，體外重組了Talin的自抑制復合物,，并進而成功解析獲得了Talin自抑制復合物的晶體結構,。結構比對發(fā)現，該復合物晶體結構與之前報道的基于NMR數據的計算模型有相當大的構象差異,，兩結構域之間的作用模式完全不同,，呈現了90度的角度差異。此外,，當形成自抑制狀態(tài)時，兩結構域朝向細胞膜一側的帶電性質完全相反,，N端子結構域為正電,，C端子結構域為負電,。
基于對該晶體結構的分析，進一步通過NMR手段對Talin突變體進行了相互作用力的研究,，并測試了溶液中細胞膜磷脂分子頭部對Talin自抑制復合物的影響,。結果顯示，細胞膜負電表面的存在很可能有助于打開Talin自抑制復合物,，從而幫助Talin解除自抑制狀態(tài),。據此，本研究對于Talin自抑制狀態(tài)的解除提出了一個新的“推－拉”模型,，基于不同靜電力的相互作用,，有別于經典的“立體沖突”模型。
本研究成果有助于深入理解Talin的調控機制,，乃至其對整合素的激活作用,。（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.
