Stefanovic Branko, Stefanovic Lela, Manojlovic Zarko
Department of Biomedical Sciences and Translational Science Laboratory, College of Medicine, Florida State University, 1115 West Call Street, Tallahassee, FL 32306, USA.
Keck School of Medicine of University of Southern California, 1450 Biggy Street, NRT 4510, Los Angeles, CA 90033, USA.
Matrix Biol Plus. 2021 Jun 23;12:100076. doi: 10.1016/j.mbplus.2021.100076. eCollection 2021 Dec.
Mechanistic aspects of type I procollagen biosynthesis in cells are poorly understood. To provide more insight into this process we designed a system to directly image type I procollagen biogenesis by co-expression of fluorescently labeled full size procollagen α1(I) and one α2(I) polypeptides. High resolution images show that collagen α1(I) and α2(I) polypeptides are produced in coordination in discrete structures on the ER membrane, which we termed the collagenosomes. Collagenosomes are disk shaped bodies, 0.5-1 μM in diameter and 200-400 nm thick, in the core of which folding of procollagen takes place. Collagenosomes are intimately associated with the ER membrane and their formation requires intact translational machinery, suggesting that they are the sites of nascent procollagen biogenesis. Collagenosomes show little co-localization with the COPII transport vesicles, which export type I procollagen from the ER, suggesting that these two structures are distinct. LARP6 is the protein which regulates translation of type I collagen mRNAs. The characteristic organization of collagenosomes depends on binding of LARP6 to collagen mRNAs. Without LARP6 regulation, collagenosomes are poorly organized and the folding of α1(I) and α2(I) polypeptides into procollagen in their cores is diminished. This indicates that formation of collagenosomes is dependent on regulated translation of collagen mRNAs. In live cells the size, number and shape of collagenosomes show little change within several hours, suggesting that they are stable structures of type I procollagen biogenesis. This is the first report of structural organization of type I collagen biogenesis in collagenosomes, while the fluorescent reporter system based on simultaneous imaging of both type I collagen polypeptides will enable the detailed elucidation of their structure and function.
细胞中I型前胶原生物合成的机制尚不清楚。为了更深入了解这一过程，我们设计了一个系统，通过共表达荧光标记的全长前胶原α1(I)和一个α2(I)多肽来直接成像I型前胶原的生物发生。高分辨率图像显示，胶原α1(I)和α2(I)多肽在ER膜上的离散结构中协同产生，我们将其称为胶原小体。胶原小体是盘状结构，直径0.5-1μm，厚200-400nm，前胶原在其核心部位发生折叠。胶原小体与ER膜紧密相连，其形成需要完整的翻译机制，这表明它们是新生前胶原生物合成的场所。胶原小体与从ER输出I型前胶原的COPII运输囊泡几乎没有共定位，这表明这两种结构是不同的。LARP6是调节I型胶原mRNA翻译的蛋白质。胶原小体的特征性组织依赖于LARP6与胶原mRNA的结合。没有LARP6的调节，胶原小体组织不良，其核心部位α1(I)和α2(I)多肽折叠成前胶原的过程减少。这表明胶原小体的形成依赖于胶原mRNA的调节翻译。在活细胞中，胶原小体的大小、数量和形状在数小时内变化不大，这表明它们是I型前胶原生物合成的稳定结构。这是关于I型胶原生物合成在胶原小体中的结构组织的首次报道，而基于同时成像两种I型胶原多肽的荧光报告系统将能够详细阐明它们的结构和功能。
