Department of Anatomy and Molecular Cell Biology, Nagoya University Graduate School of Medicine, 65 Tsurumai-cho, Showa, Nagoya 466-8550, Japan.
Mol Biol Cell. 2012 Mar;23(5):800-10. doi: 10.1091/mbc.E11-11-0950. Epub 2012 Jan 11.
Apolipoprotein B-100 (ApoB) is the principal component of very low density lipoprotein. Poorly lipidated nascent ApoB is extracted from the Sec61 translocon and degraded by proteasomes. ApoB lipidated in the endoplasmic reticulum (ER) lumen is also subjected to proteasomal degradation, but where and how it dislocates to the cytoplasm remain unknown. In the present study, we demonstrate that ApoB after lipidation is dislocated to the cytoplasmic surface of lipid droplets (LDs) and accumulates as ubiquitinated ApoB in Huh7 cells. Depletion of UBXD8, which is almost confined to LDs in this cell type, decreases recruitment of p97 to LDs and causes an increase of both ubiquitinated ApoB on the LD surface and lipidated ApoB in the ER lumen. In contrast, abrogation of Derlin-1 function induces an accumulation of lipidated ApoB in the ER lumen but does not increase ubiquitinated ApoB on the LD surface. UBXD8 and Derlin-1 bind with each other and with lipidated ApoB and show colocalization around LDs. These results indicate that ApoB after lipidation is dislocated from the ER lumen to the LD surface for proteasomal degradation and that Derlin-1 and UBXD8 are engaged in the predislocation and postdislocation steps, respectively.
载脂蛋白 B-100（ApoB）是极低密度脂蛋白的主要成分。新生的疏水性载脂蛋白 B 从 Sec61 转位器中被提取出来，并被蛋白酶体降解。内质网腔中脂化的 ApoB 也会被蛋白酶体降解，但它如何以及在哪里移位到细胞质中仍然未知。在本研究中，我们证明了脂化后的 ApoB 被移位到脂滴（LDs）的细胞质表面，并作为泛素化的 ApoB 在 Huh7 细胞中积累。UBXD8 的耗竭几乎局限于该细胞类型中的 LDs，这会减少 p97 向 LDs 的募集，并导致 LD 表面上的泛素化 ApoB 和 ER 腔中的脂化 ApoB 增加。相比之下，Derlin-1 功能的缺失会导致 ER 腔中脂化的 ApoB 积累，但不会增加 LD 表面上的泛素化 ApoB。UBXD8 和 Derlin-1 相互结合，并与脂化的 ApoB 结合，并在 LD 周围显示共定位。这些结果表明，脂化后的 ApoB 从 ER 腔移位到 LD 表面进行蛋白酶体降解，而 Derlin-1 和 UBXD8 分别参与预移位和后移位步骤。
