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(ER)应激诱导的膜蛋白，具有泛素样域(ULD)。然而，它的生物学功能尚不清楚。以前，我们曾报道HERP在细胞中的高表达促进淀粉样蛋白β-蛋白(Aβ)的产生，并且HERP与早老素(PS)相互作用。β是由淀粉样前体蛋白(APP)在β-和γ-分泌酶的催化下，通过一系列的蛋白水解酶而产生的。PS通过内蛋白水解法被加工成稳定的形式。已加工PS的细胞水平受到高度调控，过量的未加工PS进入泛素/蛋白酶体降解途径。加工后的PS存在于一个高相对分子质量的复合体中，该复合体包括三种膜蛋白，即Nacstrin(NCT)、APH-1和PEN-2。越来越多的证据表明，PS复合体与γ-分泌酶活性有关。在这项研究中，我们讨论了HERP在PS复合体形成中的调节作用。我们发现，ULD缺失的Herp的表达显著抑制了未处理的PS或NCT的降解，这些PS或NCT未能被结合到PS复合体中。我们还发现，ULD缺失的Herp基因的表达在一定程度上降低了PS和NCT的泛素化，而野生型Herp的表达增强了PS和NCT的泛素化，强烈表明Herp的ULD参与了PS和NCT的泛素化。因此，Herp很可能通过其泛素样结构域，在消除多余的PS或PS辅助因子方面发挥作用，这些PS或PS辅助因子无法驻留在PS复合体中，从而调节PS复合体的细胞内水平。

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.