Department of Biomedical Sciences, College of Medicine, Florida State University, Tallahassee, Florida 32306, USA.
Sci Rep. 2017 Jan 23;7:41173. doi: 10.1038/srep41173.
Excessive deposition of type I collagen causes fibrotic diseases. Binding of La ribonucleoprotein domain family, member 6 (LARP6) to collagen mRNAs regulates their translation and is necessary for high type I collagen expression. Here we show that mTORC1 phosphorylates LARP6 on S348 and S409. The S348A/S409A mutant of LARP6 acts as a dominant negative protein in collagen biosynthesis, which retards secretion of type I collagen and causes excessive posttranslational modifications. Similar effects are seen using mTORC1 inhibitor rapamycin or by knocking down raptor. The S348A/S409A mutant weakly interacts with the accessory protein STRAP, needed for coordinated translation of collagen mRNAs. The interaction of wt LARP6 and STRAP is also attenuated by rapamycin and by raptor knockdown. Additionally, in the absence of S348/S409 phosphorylation LARP6 is sequestered in increasing amounts at the ER membrane. We postulate that phosphorylation of S348/S409 by mTORC1 stimulates the interaction of LARP6 and STRAP to coordinate translation of collagen mRNAs and to release LARP6 from the ER for new round of translation. These mechanisms contribute to high level of collagen expression in fibrosis.
Ⅰ型胶原过度沉积会导致纤维化疾病。La 核糖核蛋白结构域家族成员 6（LARP6）与胶原 mRNA 的结合调节其翻译，这对于Ⅰ型胶原的高表达是必需的。在这里，我们发现 mTORC1 可使 LARP6 的 S348 和 S409 发生磷酸化。LARP6 的 S348A/S409A 突变体在胶原生物合成中充当显性负性蛋白，可延迟Ⅰ型胶原的分泌并导致其过度翻译后修饰。使用 mTORC1 抑制剂雷帕霉素或敲低 raptor 也会产生类似的效果。S348A/S409A 突变体与辅助蛋白 STRAP 的相互作用较弱，STRAP 对于胶原 mRNA 的协调翻译是必需的。雷帕霉素和 raptor 敲低也会减弱 wt LARP6 和 STRAP 的相互作用。此外，在缺乏 S348/S409 磷酸化的情况下，LARP6 会以越来越多的量被隔离在 ER 膜上。我们推测 mTORC1 对 S348/S409 的磷酸化可刺激 LARP6 和 STRAP 的相互作用，从而协调胶原 mRNA 的翻译，并将 LARP6 从 ER 中释放出来以进行新一轮的翻译。这些机制有助于纤维化中胶原的高水平表达。
