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Maintenance of endoplasmic reticulum (ER) proteostasis is controlled by a dynamic signaling network known as the unfolded protein response (UPR). IRE1α is a major UPR transducer, determining cell fate under ER stress. We used an interactome screening to unveil several regulators of the UPR, highlighting the ER chaperone Hsp47 as the major hit. Cellular and biochemical analysis indicated that Hsp47 instigates IRE1α signaling through a physical interaction. Hsp47 directly binds to the ER luminal domain of IRE1α with high affinity, displacing the negative regulator BiP from the complex to facilitate IRE1α oligomerization. The regulation of IRE1α signaling by Hsp47 is evolutionarily conserved as validated using fly and mouse models of ER stress. Hsp47 deficiency sensitized cells and animals to experimental ER stress, revealing the significance of Hsp47 to global proteostasis maintenance. We conclude that Hsp47 adjusts IRE1α signaling by fine-tuning the threshold to engage an adaptive UPR.

Extracellular matrix (ECM) proteins are biosynthesized in the rough endoplasmic reticulum (rER) and transported via the Golgi apparatus to the extracellular space. The coat protein complex II (COPII) transport vesicles are approximately 60-90 nm in diameter. However, several ECM molecules are much larger, up to several hundreds of nanometers. Therefore, special COPII vesicles are required to coat and transport these molecules. Transmembrane Protein Transport and Golgi Organization 1 (TANGO1) facilitates loading of collagens into special vesicles. The Src homology 3 (SH3) domain of TANGO1 was proposed to recognize collagen molecules, but how the SH3 domain recognizes various types of collagen is not understood. Moreover, how are large noncollagenous ECM molecules transported from the rER to the Golgi? Here we identify heat shock protein (Hsp) 47 as a guide molecule directing collagens to special vesicles by interacting with the SH3 domain of TANGO1. We also consider whether the collagen secretory model applies to other large ECM molecules.E xtracellular matrix (ECM) proteins are the most abundant proteins in our body. They build the body architecture and help to maintain tissue homeostasis in bone, skin, cartilage, and tendon (1). They are biosynthesized in the rough endoplasmic reticulum (ER; rER) and traverse the Golgi apparatus en route to the extracellular space. A transport vesicle for cargo exists to traffic molecules from the rER into the extracellular space. This secretory vesicle is called a coat protein complex II (COPII) vesicle (2-4). It is generated at the ER exit site and usually has a diameter of 60-90 nm. This carrier can transport many secreted proteins. However, larger or elongated molecules, especially collagens and other ECM molecules, will not fit into such small vesicles.There are more than 20 different collagen types in humans (5, 6). Type I, II, III, and V collagens are classified as fibrillar collagens, which are the most abundant. Type IV collagen is an important structural element in basement membranes. Type VII collagen forms anchoring fibrils at the epidermal-dermal junction in skin. These collagens are all more than 300 nm long. In addition to its elongated triple helical structure, type VII collagen has a 140-kDa noncollagenous domain at the amino terminus. Although more flexible than collagens, fibrillin molecules are also elongated, measured to be approximately 150 nm in length (7). For these molecules, special COPII vesicles or other trafficking systems are required for secretion.It has been proposed that procollagen molecules, which are the precursors of collagen, are transferred in COPII vesicles to the Golgi (8-10). In 2009, Transmembrane Protein Transport and Golgi Organization 1 (TANGO1), encoded by MIA3, was shown to facilitate collagens into COPII vesicles at the ER exit site (11). In the cytoplasm, TANGO1 interacts with Sec23A/Sec24C, the outer molecules of COPII vesicles, and cTAGE5, a homolog of TANGO1 that lacks the rER domains of TANGO. These interactions occur via a ...

Heat shock protein 47 (Hsp47) is an endoplasmic reticulum (ER)-resident molecular chaperone essential for correct folding of procollagen in mammalian cells. In this Review, we discuss the role and function of Hsp47 in vertebrate cells and its role in connective tissue disorders. Hsp47 binds to collagenous (Gly-Xaa-Arg) repeats within triple-helical procollagen in the ER and can prevent its local unfolding or aggregate formation, resulting in accelerating triple-helix formation of procollagen. Hsp47 pH-dependently dissociates from procollagen in the cis-Golgi or ER-Golgi intermediate compartment and is then transported back to the ER. Although Hsp47 belongs to the serine protease inhibitor (serpin) superfamily, it does not possess serine protease inhibitory activity. Whereas general molecular chaperones such as Hsp70 and Hsp90 exhibit broad substrate specificity, Hsp47 has narrower specificity mainly for procollagens. However, other Hsp47-interacting proteins have been recently reported, suggesting a much broader role for Hsp47 in the cell that warrants further investigation. Other ER-resident stress proteins, such as binding immunoglobulin protein (BiP), are induced by ER stress, whereas Hsp47 is induced only by heat shock. Constitutive expression of Hsp47 is always correlated with expression of various collagen types, and disruption of the Hsp47 gene in mice causes embryonic lethality due to impaired basement membrane and collagen fibril formation. Increased Hsp47 expression is associated with collagen-related disorders such as fibrosis, characterized by abnormal collagen accumulation, highlighting Hsp47's potential as a clinically relevant therapeutic target. cro REVIEWS

Fibrosis can disrupt tissue structure and integrity and impair organ function. Fibrosis is characterized by abnormal collagen accumulation in the extracellular matrix. Pharmacological inhibition of collagen secretion therefore represents a promising strategy for the management of fibrotic disorders, such as liver and lung fibrosis. Hsp47 is an endoplasmic reticulum (ER)-resident collagen-specific molecular chaperone essential for correct folding of procollagen in the ER. Genetic deletion of Hsp47 or inhibition of its interaction with procollagen interferes with procollagen triple helix production, which vastly reduces procollagen secretion from fibroblasts. Thus, Hsp47 could be a potential and promising target for the management of fibrosis. In this study, we screened small-molecule compounds that inhibit the interaction of Hsp47 with collagen from chemical libraries using surface plasmon resonance (BIAcore), and we found a molecule AK778 and its cleavage product Col003 competitively inhibited the interaction and caused the inhibition of collagen secretion by destabilizing the collagen triple helix. Structural information obtained with NMR analysis revealed that Col003 competitively binds to the collagen-binding site on Hsp47. We propose that these structural insights could provide a basis for designing more effective therapeutic drugs for managing fibrosis.