Procollagen quality control at ER exit sites
Type I collagen is the main component of bone and other connective tissues. Defects in procollagen folding, trafficking and degradation are key factors in bone and connective tissue pathologies. Autophagy has been identified as the degradation pathway for misfolded procollagen that avoids activation of generic ER-stress responses; however, the mechanism by which this occurs is unclear. To address this question, we used live cell and correlative light-electron microscopy (CLEM) imaging of procollagen with a Gly610-to-Cys substitution in mouse osteoblasts. This substitution mimics a mouse model of bone pathology that involves autophagic degradation of misfolded mutant procollagen. Cells expressing fluorescently tagged proteins confirmed that some dynamic procollagen puncta (~500nm) departed from ER exit sites (ERESs) and traveled on to Golgi whereas other puncta co-localized with autophagic structures. These two fractions of procollagen represent the secretory compartment and the degradative compartment, respectively. Although procollagen vesicular traffic departing ERESs lacked components of the COPII coat (proteins associated with ER exit site formation), procollagen autophagic puncta colocalized with these ERES components, but not with ER lumen or membrane proteins. Moreover, when ERES formation was inhibited procollagen autophagic puncta decreased. This suggests that the rerouting of misfolded procollagen to autophagy occurs at ERESs. Further analyses, including CLEM, revealed that this delivery occurred via a non-traditional form of autophagy known as microautophagy, involving lysosomal engulfment of autophagic LC3-labeled ERESs containing misfolded procollagen. Our current studies focus on the mechanisms involved in the sorting of folded vs misfolded procollagen at the level of the ERES.
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