p21蛋白質是細胞週期蛋白依賴性激素 (cyclin dependent kinase，CDK) 的有效抑制劑。p21蛋白質可以和細胞週期蛋白 (cyclin)、細胞週期蛋白依賴性激素 (cyclin dependent kinase) 以及增生細胞核抗原 (proliferating cell nuclear antigen, PCNA) 結合。然而，最近有許多新的p21交互作用蛋白被發現。例如，p21蛋白可以結合procapase-3，進而抑制caspase-3的活化。在這個實驗中有兩個主要的目的，一個是獲得足夠的p21蛋白去解它的結構，另一個是去探討p21的交互作用的蛋白質。我們從細菌中獲得大量的重組p21蛋白。起初那些蛋白是以inclusion body 存在，但是我們利用一個已發表的方法去處理它們，使它們變性而後摺疊回來。圓二色光譜 (circular dichroism spectroscopy) 的數據證明摺疊回來的p21蛋白有α-helix 的二級結構。並且藉由免疫沉澱 (immunoprecipitation) 和Far-Western的方法證實這個摺疊回來的p21 蛋白可以和PCNA，CDK2，cyclin D1 結合。而後我們在中國倉鼠卯巢細胞中發現一些蛋白會和p21結合，但是目前它們的身份還沒被鑑別出來。結果顯示這個重組的p21蛋白是適合用來研究上述我們想要研究的課題。

The p21 (Waf1/Cip1) protein is a general inhibitor of cyclin dependent kinase (CDK). Although it has been known that p21 interacts with CDK, cyclin D1 and proliferating cell nuclear antigen (PCNA), recent studies have found that p21 interacts with many other proteins. For example, p21 can interact with procaspase-3 to inhibit caspase-3 activation. The aims of the study are two; to obtain enough p21 protein for structural analysis, and, to explore the novel p21 interacting proteins. A large quantity of recombinant human p21 protein was obtained from E. coli. Although the proteins were initially collected in the form of inclusion bodies, they were renatured through a published scheme. Circular dichroism spectroscopy shows that the refolded proteins acquired helical structure at the similar fraction as predicted from the primary sequence of the protein. Immunoprecipitation and Far-Western analyses show that the refolded p21 protein bound with PCNA, CDK2, and cyclin D1. Some proteins of the CHO-K1 cells were found to interact with the p21 protein, although their identities remain unknown. These results suggest that the recombinant p21 protein is useful to pursuit the aims described earlier.

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