突觸結合蛋白 I (Synaptotagmin I, Syt I)是神經細胞突觸囊泡上的一種膜蛋白，而C2A與C2B則是其具有重要功能的胞質片段。近年的研究顯示，C2A會與鈣離子作用，而C2B則會與多磷酸肌醇作用。在鈣離子引發的神經傳導物質釋放的過程中，C2A與鈣離子的結合會活化神經傳導物質從神經突觸前膜釋放，但當C2B與多磷酸肌醇結合則會抑制此過程。利用親和性層析方法先將C2AB片段與老鼠大腦萃取物混合後，再以含有六磷酸肌醇之沖湜液沖湜，發現另一複合蛋白的次體、μ2, the subunit of clathrin assembly protein, AP-2。此結果顯示，當六磷酸肌醇與C2B結合時，會改變Syt I–AP-2間的作用力，進而抑制神經傳導物質的釋放。我們利用多維核磁共振的技術來研究C2B與六磷酸肌醇的複合物結構。我們的研究結果顯示，當六磷酸肌醇與C2B結合後，會引發C2B蛋白之結構改變，進而使C2B喪失了與AP-2的結合力。此研究結果在藥物學的治療上針對神經失調疾病將有助於開發出更好的新藥。

Abstract
Synaptotagmin I (Syt I) is a synaptic vesicle membrane protein that contains two copies of highly conserved protein kinase C homology regions known as the C2A and C2B domains. The C2A domain binds Ca2+ and the C2B domain binds inositol high polyphosphates (IP4, IP5, and IP6). It has been reported that Ca2+ regulated exocytosis of secretory vesicles is proposed to be activated by Ca2+ binding to the C2A domain and inhibited by inositol polyphosphate binding to the C2B domain. Syt I is also known to be present in neuronal growth cone vesicles. Affinity elution chromatography from the C2 domain of Syt I-immobilized Sepharose using IP6 as the eluent found that several proteins, including an adaptin, specific subunits of the clathrin assembly protein, AP-2 were eluted from the mouse brain. It suggests that inositol high polyphosphate-binding to the C2B domain of Syt I alter the state of protein–protein interaction including the Syt I–AP-2 interaction. Thus, the inositol high polyphosphate may result in the inhibition of events involved in the synaptic vesicle trafficking. In this study, we used NMR to solve the C2B-IP6 3D complex structure. Our data provide new evidence for the hypothesis that the conformational change of C2B binding to IP6 alter the interface of C2B- AP-2. This information will aid in the design of better pharmacological treatments for neurological disorders.

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