Activation and substrate recognition mechanism of calpain

钙蛋白酶是一种细胞内钙依赖性半胱氨酸蛋白酶，它通过在一定程度上切割底物蛋白，改变或下调其活性或功能来发挥多种生理功能。虽然钙蛋白酶的结构和生化特征已经被很好地确定，底物特异性或底物识别机制仍有待研究。钙蛋白酶抑制素是一种内源性钙蛋白酶抑制剂，与钙蛋白酶一样分布广泛。钙蛋白酶和钙蛋白酶抑制素之间的相互作用是非常特异的，因此这将是一个很好的模型，底物识别钙蛋白酶。研究了钙蛋白酶抑制蛋白肽对钙离子和其他金属离子引起的钙蛋白酶结构变化的影响。这些肽被认为结合在钙蛋白酶的疏水表面，通过钙离子的结合而暴露。钙蛋白酶的底物膜联蛋白I的晶体结构也得到了解决。钙蛋白酶在膜联蛋白I的N-尾切割的位点没有足够的密度来建立模型。完善工作仍在进行中。

Calpain, an itracelular calcium-dependent cystein protease, is thought to perform various physiological functions by cleaving substrate proteins to a limited extent, and altering or down-regulating their activities or functions. Altough the structural and biochemical features of calpains have been well defined, substrate specificity or substrate-recognition mechanism remains to be investigated. Calpastatin, an endogenous calpain inhibitor is distributed ubiquitously as well as calpain. Interaction between calpain and calpastatin is very specific, so this will be a good model for substrate recognition by calpain. Effects of calpastatin peptides against structural changes of calpain induced by calcium ions and other metal ions were investigated. The peptides supposed to bind at hydrophobic surface of calpain exposed by the binding of calcium ions. Crystal structure of annexin I,a substrate of calpain, was also solved. The site cleaved by calpain at N-tail of annexin I did not have enough density to build a model. The refinement is still in progress.