Expression of the CDH1-associated form of the anaphase-promoting complex in postmitotic neurons

The anaphase-promoting complex/cyclosome (APC) is a tightly cell cycle-regulated ubiquitin-protein ligase that targets cyclin B and other destruction box-containing proteins for proteolysis at the end of mitosis and in G1. Recent work has shown that activation of the APC in mitosis depends on CDC20, whereas APC is maintained active in G1 via association with the CDC20-related protein CDH1. Here we show that the mitotic activator CDC20 is the only component of the APC ubiquitination pathway whose expression is restricted to proliferating cells, whereas the APC and CDH1 are also expressed in several mammalian tissues that predominantly contain differentiated cells, such as adult brain. Immunocytochemical analyses of cultured rat hippocampal neurons and of mouse and human brain sections indicate that the APC and CDH1 are ubiquitously expressed in the nuclei of postmitotic terminally differentiated neurons. The APC purified from brain contains all core subunits known from proliferating cells and is tightly associated with CDH1. Purified brain APC(CDH1) has a high cyclin B ubiquitination activity that depends less on the destruction box than on the activity of mitotic APC(CDC20). On the basis of these results, we propose that the functions of APC(CDH1) are not restricted to controlling cell-cycle progression but may include the ubiquitination of yet unidentified substrates in differentiated cells.

后期促进复合物/周期体（anaphase-promoting complex/cyclosome, APC）是一类受细胞周期严格调控的泛素蛋白连接酶，可靶向细胞周期蛋白B（cyclin B）及其他携带破坏盒（destruction box）的蛋白质，在有丝分裂末期及G1期介导蛋白水解。近期研究证实，有丝分裂阶段APC的激活依赖于CDC20，而APC在G1期的持续活化则通过与CDC20同源蛋白CDH1结合得以维持。本研究表明，作为有丝分裂激活因子的CDC20，是APC泛素化通路中唯一仅在增殖细胞中表达的组分；而APC与CDH1在多种以分化细胞为主的哺乳动物组织中均有表达，例如成年脑组织。对体外培养的大鼠海马神经元以及小鼠、人脑组织切片开展的免疫细胞化学分析显示，APC与CDH1广泛表达于有丝分裂后终末分化神经元的细胞核内。从脑组织中纯化获得的APC包含增殖细胞中已鉴定的全部核心亚基，并与CDH1紧密结合。经纯化的脑组织APC(CDH1)具备较高的细胞周期蛋白B泛素连接活性，相较于有丝分裂APC(CDC20)，其活性对破坏盒的依赖性更低。基于上述实验结果，我们提出APC(CDH1)的功能并非仅局限于调控细胞周期进程，还可能参与对分化细胞中尚未被鉴定的底物进行泛素化修饰。