@article {65631,
	title = {Mechanisms of procollagen and HSP47 sorting during ER-to-Golgi trafficking},
	journal = {Matrix Biology},
	volume = {93},
	year = {2020},
	month = {2020 Nov 01},
	pages = {79-94},
	abstract = {<p>Efficient quality control and export of procollagen from the cell is crucial for extracellular matrix homeostasis, yet it is still incompletely understood. One of the debated questions is the role of a collagen-specific ER chaperone HSP47 in these processes. Most ER chaperones preferentially bind to unfolded polypeptide chains, enabling selective export of natively folded proteins from the ER after chaperone release. In contrast, HSP47 preferentially binds to the natively folded procollagen and is believed to be released only in the ER-Golgi intermediate compartment (ERGIC) or cis-Golgi. HSP47 colocalization with procollagen in punctate structures observed by immunofluorescence imaging of fixed cells has thus been interpreted as evidence for HSP47 export from the ER together with procollagen in transport vesicles destined for ERGIC or Golgi. To understand the mechanism of this co-trafficking and its physiological significance, we imaged the dynamics of fluorescently tagged type I procollagen and HSP47 punctate structures in live MC3T3 murine osteoblasts with up to 120 nm spatial and 500 ms time resolution. Contrary to the prevailing model, we discovered that most bona fide carriers delivering procollagen from ER exit sites (ERESs) to Golgi contained no HSP47, unless the RDEL signal for ER retention in HSP47 was deleted or mutated. These transport intermediates exhibited characteristic rapid, directional motion along microtubules, while puncta with colocalized HSP47 and procollagen similar to the ones described before had only limited, stochastic motion. Live cell imaging and fluorescence recovery after photobleaching revealed that the latter puncta (including the ones induced by ARF1 inhibition) were dilated regions of ER lumen, ERESs, or autophagic structures surrounded by lysosomal membranes. Procollagen was colocalized with HSP47 and ERGIC53 at ERESs. It was colocalized with ERGIC53 but not HSP47 in Golgi-bound transport intermediates. Our results suggest that procollagen and HSP47 sorting occurs at ERES before procollagen is exported from the ER in Golgi-bound transport intermediates, providing new insights into mechanisms of procollagen trafficking.</p>
},
	issn = {0945053X},
	doi = {10.1016/j.matbio.2020.06.002},
	url = {https://linkinghub.elsevier.com/retrieve/pii/S0945053X20300676},
	author = {Omari, Shakib and Makareeva, Elena and Gorrell, Laura and Jarnik, Michal and Lippincott-Schwartz, Jennifer and Leikin, Sergey}
}
@article {49457,
	title = {Noncanonical autophagy at ER exit sites regulates procollagen turnover.},
	journal = {Proceedings of the National Academy of Sciences of the United States of America},
	volume = {115},
	year = {2018},
	month = {2018 Oct 04},
	pages = {E10099-108},
	abstract = {<p>Type I collagen is the main component of bone matrix and other connective tissues. Rerouting of its procollagen precursor to a degradative pathway is crucial for osteoblast survival in pathologies involving excessive intracellular buildup of procollagen that is improperly folded and/or trafficked. What cellular mechanisms underlie this rerouting remains unclear. To study these mechanisms, we employed live-cell imaging and correlative light and electron microscopy (CLEM) to examine procollagen trafficking both in wild-type mouse osteoblasts and osteoblasts expressing a bone pathology-causing mutant procollagen. We found that although most procollagen molecules successfully trafficked through the secretory pathway in these cells, a subpopulation did not. The latter molecules appeared in numerous dispersed puncta colocalizing with COPII subunits, autophagy markers and ubiquitin machinery, with more puncta seen in mutant procollagen-expressing cells. Blocking endoplasmic reticulum exit site (ERES) formation suppressed the number of these puncta, suggesting they formed after procollagen entry into ERESs. The punctate structures containing procollagen, COPII, and autophagic markers did not move toward the Golgi but instead were relatively immobile. They appeared to be quickly engulfed by nearby lysosomes through a bafilomycin-insensitive pathway. CLEM and fluorescence recovery after photobleaching experiments suggested engulfment occurred through a noncanonical form of autophagy resembling microautophagy of ERESs. Overall, our findings reveal that a subset of procollagen molecules is directed toward lysosomal degradation through an autophagic pathway originating at ERESs, providing a mechanism to remove excess procollagen from cells.</p>
},
	issn = {1091-6490},
	doi = {10.1073/pnas.1814552115},
	author = {Omari, Shakib and Makareeva, Elena and Roberts-Pilgrim, Anna and Mirigian, Lynn and Jarnik, Michal and Ott, Carolyn and Lippincott-Schwartz, Jennifer and Leikin, Sergey}
}
