Analysis of the role of Herp in the generation of amyloid β protein and in the ER stress

Herp是一种内质网应激诱导的膜蛋白，具有泛素样结构域（ULD）。但其生物学功能尚不清楚。以前，我们报道了Herp在细胞中的高表达水平增强了淀粉样β蛋白（Aβ）的产生，并且Herp与早老素（PS）相互作用。Aβ由淀粉样前体蛋白（APP）通过β-和γ-分泌酶催化的连续蛋白水解裂解产生。PS通过内蛋白水解被加工成其稳定形式。加工的PS的细胞水平受到高度调节，并且过量的未加工的PS进入泛素/蛋白酶体降解途径。加工后的PS存在于高分子量复合物中，该复合物包括三种膜蛋白，即NCT、APH-1和PEN-2。越来越多的证据表明，PS复合物负责γ-分泌酶的活性。在这项研究中，我们解决的作用，Herp在PS复合物的形成的调节。我们发现，ULD缺失的Herp的表达显着抑制未加工的PS或NCT，未能纳入PS复合物的降解。我们还发现，ULD缺失的Herp的表达降低了PS和NCT的泛素化程度，而野生型Herp的表达增强了它，强烈表明Herp的ULD参与了PS和NCT的泛素化。因此，Herp可能通过其泛素样结构域在消除未能驻留在PS复合物中的过量PS或PS辅因子中发挥作用，从而调节PS复合物的细胞内水平。

Herp is an endoplasmic reticulum (ER)-stress-inducible membrane protein that has a ubiquitin-like domain (ULD). However, its biological function is not yet known. Previously, we reported that a high expression level of Herp in cells enhances the generation of amyloid β-protein (Aβ) and that Herp interacts with presenilin (PS). Aβ is generated from amyloid precursor protein (APP) by sequential proteolytic cleavages catalyzed by β- and γ-secretases. PS is processed to its stable form by endoproteolysis. The cellular level of processed PS is highly regulated, and excess unprocessed PS enters the ubiquitin/proteasome degradation pathway. Processed PS resides in a high-molecular-weight complex that includes three membrane proteins, namely, Nacastrin (NCT), APH-1 and PEN-2. Accumulating evidence shows that the PS complex is responsible for y-secretase activity. In this study, we addressed the role of Herp in the regulation of PS complex formation. We found that the expression of ULD-deleted Herp markedly inhibited the degradation of unprocessed PS or NCT that fails to be incorporated into the PS complex. We also found that the expression of ULD-deleted Herp decreased a degree of ubiqutination of PS and NCT, while the expression of wild-type Herp enhanced it, strongly suggesting that ULD of Herp is involved for the ubiquitination of PS and NCT. Thus, Herp is likely to play a role, through its ubiquitin-like domain, in the elimination of excess PS or PS cofactors which fail to reside in the PS complex, thereby regulating the intracellular level of the PS complex.