Kato Hiroki, Sakaki Kenjiro, Mihara Katsuyoshi
Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka, Japan.
J Cell Sci. 2006 Jun 1;119(Pt 11):2342-53. doi: 10.1242/jcs.02951.
Mammalian Delta9 stearoyl-CoA desaturase 1 (SCD1) is a key enzyme in the biosynthesis of mono-unsaturated fatty acids in the endoplasmic reticulum (ER). It is a short-lived multispanning ER membrane protein, reported to be degraded by the ubiquitin-proteasome-independent pathway. We have examined SCD1 protein degradation using cultured mammalian cells. Exogenously expressed SCD1 in CHO-K1 cells was localized to the ER and turned over with a half-life of approximately 3 hours. Unexpectedly, proteasome inhibitors increased the half-life of SCD1 to approximately 6 hours. Endogenously expressed SCD1 in adipocyte-differentiated NIH 3T3-L1 cells was also rapidly degraded in a proteasome inhibitor-sensitive manner. In the presence of proteasome inhibitors, polyubiquitylated SCD1 accumulated in the ER and interacted with AAA-ATPase p97, which is involved in ER-associated degradation (ERAD). The 66-residue N-terminal segment carrying the PEST sequence is mainly responsible for SCD1 degradation and this segment induced instability in an otherwise stable ER membrane protein. Furthermore, SCD1 was degraded constitutively irrespective of the cellular levels of unsaturated fatty acids, which strictly regulate SCD1 gene expression. These findings indicate that the ubiquitin-proteasome-dependent ERAD system is also involved in constitutive SCD1 degradation.
哺乳动物Δ9硬脂酰辅酶A去饱和酶1（SCD1）是内质网（ER）中单不饱和脂肪酸生物合成的关键酶。它是一种寿命较短的多跨膜内质网蛋白，据报道可通过不依赖泛素-蛋白酶体的途径降解。我们使用培养的哺乳动物细胞研究了SCD1蛋白的降解。在CHO-K1细胞中外源表达的SCD1定位于内质网，其半衰期约为3小时。出乎意料的是，蛋白酶体抑制剂将SCD1的半衰期延长至约6小时。在脂肪细胞分化的NIH 3T3-L1细胞中内源性表达的SCD1也以蛋白酶体抑制剂敏感的方式迅速降解。在蛋白酶体抑制剂存在的情况下，多聚泛素化的SCD1在内质网中积累，并与参与内质网相关降解（ERAD）的AAA-ATP酶p97相互作用。携带PEST序列的66个残基的N末端片段主要负责SCD1的降解，并且该片段在原本稳定的内质网膜蛋白中诱导不稳定性。此外，无论不饱和脂肪酸的细胞水平如何，SCD1都会被组成型降解，而不饱和脂肪酸的细胞水平严格调节SCD1基因的表达。这些发现表明，依赖泛素-蛋白酶体的内质网相关降解系统也参与了SCD1的组成型降解。
