Broadly Conserved TMEM-131 Family Proteins in Collagen Production and Secretory Cargo Trafficking

Abstract

Collagen is the most abundant protein in animals. Its dysregulation contributes to ageing and human disorders including tissue fibrosis in major organs. How premature collagens in the endoplasmic reticulum (ER) assemble and route for secretion remains molecularly undefined. From an RNAi screen, we identified an uncharacterized C. elegans gene tmem-131, deficiency of which impairs collagen production and activates ER stress response. TMEM-131 N-termini contain bacterial PapD chaperone-like (PapD-L) domains essential for collagen assembly and secretion. Human TMEM131 binds to COL1A2 and TRAPPC8 via N-terminal PapD-L and C-terminal domain, respectively, to drive collagen production. We provide evidence that previously undescribed roles of TMEM131 in collagen recruitment and secretion are evolutionarily conserved in C. elegans, Drosophila and humans.Collagen is the major extracellular component of connective tissues and is the most abundant protein in animals (1-3). Production of mature collagen is a multi-step process, involving collagen gene regulation, protein biosynthesis, post-translational modifications in the endoplasmic reticulum (ER), formation of secretion-competent trimers, and extracellular C-propeptide cleavage and cross-linking among trimers (1-3). Dysregulation of collagen production or deposition contributes to a wide variety of human disorders, including diabetes, ageing and pathological tissue fibrosis in major organs such as kidneys, liver, lungs and heart (4-9). The type I collagen is the most abundant fibril-type collagen and its trimer comprises two α1 (I) procollagen chains and one α2 (I) procollagen chain, encoded by the genes COL1A1 and COL1A2, respectively. Both COL1A1 and COL1A2 contain C-terminal domains (C-propeptide) responsible for initial chain trimerization. The trimerization occurs via a zipper-like mechanisms, initiating from the C-propeptide domain of α1/2 (I) in close proximity to ER membranes (10-13). Although the enzymes responsible for type I collagen modification, extracellular cleavage and cross-linking have been well described (1,14), how procollagen monomers are recruited to assemble into secretioncompetent multimers in the ER remains poorly understood.COPII-coated vesicles mediate ER-to-Golgi anterograde transport of secretioncompetent cargos, including those containing collagens (15,16). Tethering COPII vesicles from ER to Golgi membranes requires TRAPP (transport protein particle), a multi-subunit protein complex highly conserved in eukaryotes (17,18). TRAPPC8 is a key component of TRAPP III, a subtype of TRAPP that acts as a guanine nucleotide exchange factor (GEF) to activate Rab GTPase to promote ER-to-Golgi cargo trafficking. Collagen production also requires HSP47, a procollagen chaperone in the ER, and TANGO1, an ER transmembrane protein that facilitates the export of bulky cargo with collagen (19,20).Intriguingly, HSP47 and TANGO1 orthologs are present only in vertebrates (21-23), raising the question whether mechanisms conserved in all animals exist to recognize ...

ER-GUARD: an evolutionarily conserved antioxidant defense system at ER membranes

“…2C). Furthermore, we assayed effects of ergu-1 null, protein-truncating mutants or RNAi on a comprehensive panel of stress-responding reporters, cell typespecific markers, and organelle membrane lipid sensors (26)(27)(28), but did not observe apparent phenotypic consequences, except oxidative stress-related reporters gst-4p::GFP, roGFP2-Orp1 and Grx1-roGFP2 (Fig. S2 and S3).…”