Hsp40s play distinct roles during the initial stages of apolipoprotein B biogenesis.

Apolipoprotein B (ApoB) is the primary component of atherogenic lipoproteins, which transport serum fats and cholesterol. Therefore elevated levels of circulating ApoB are a primary risk factor for cardiovascular disease. During ApoB biosynthesis in the liver and small intestine under nutrient-rich conditions, ApoB cotranslationally translocates into the endoplasmic reticulum (ER) and is lipidated and ultimately secreted. Under lipid-poor conditions, ApoB is targeted for ER-associated degradation (ERAD). Although prior work identified select chaperones that regulate ApoB biogenesis, the contributions of cytoplasmic Hsp40s are undefined. To this end, we screened ApoB-expressing yeast and determined that a class A ER-associated Hsp40, Ydj1, associates with and facilitates the ERAD of ApoB. Consistent with these results, a homologous Hsp40, DNAJA1, functioned similarly in rat hepatoma cells. DNAJA1-deficient cells also secreted hyperlipidated lipoproteins in accordance with attenuated ERAD. In contrast to the role of DNAJA1 during ERAD, DNAJB1—a class B Hsp40—helped stabilize ApoB. Depletion of DNAJA1 and DNAJB1 also led to opposing effects on ApoB ubiquitination. These data represent the first example in which different Hsp40s exhibit disparate effects during regulated protein biogenesis in the ER and highlight distinct roles that chaperones can play on a single ERAD substrate.

载脂蛋白B（ApoB）是致动脉粥样硬化脂蛋白的主要成分，这些脂蛋白运输血清脂肪和胆固醇。因此，循环中ApoB水平升高是心血管疾病的一个主要危险因素。在营养丰富的条件下，肝脏和小肠中ApoB的生物合成过程中，ApoB共翻译转运进入内质网（ER），被脂化并最终分泌。在脂质缺乏的条件下，ApoB被靶向内质网相关降解（ERAD）。尽管先前的研究确定了一些调节ApoB生物发生的伴侣蛋白，但细胞质热休克蛋白40（Hsp40）的作用尚未明确。为此，我们对表达ApoB的酵母进行了筛选，并确定了一种A类内质网相关的Hsp40，Ydj1，与ApoB结合并促进其内质网相关降解。与这些结果一致，一种同源的Hsp40，DNAJA1，在大鼠肝癌细胞中具有类似的功能。DNAJA1缺陷细胞也会因内质网相关降解减弱而分泌高脂化的脂蛋白。与DNAJA1在内质网相关降解中的作用相反，DNAJB1——一种B类Hsp40——有助于稳定ApoB。DNAJA1和DNAJB1的缺失也对ApoB的泛素化产生相反的影响。这些数据代表了第一个不同的Hsp40在内质网中调节蛋白质生物发生过程中表现出不同作用的例子，并突出了伴侣蛋白在单个内质网相关降解底物上可以发挥的不同作用。