Fig. 10.

Noncanonical ERES microautophagy model of procollagen degradation. Schematics of macroautophagy (A) and microautophagy (B) pathways of ERES degradation. In macroautophagy, the cargo is first internalized inside a double-membrane autophagosome followed by autophagosome-lysosome fusion. In microautophagy, the cargo is directly engulfed by a lysosome. Misfolded procollagen aggregates appear to enter ERESs on their own or together with normally folded molecules, preventing formation of Golgi-bound carrier vesicles and activating autophagy machinery (perhaps because of their size/structure). These ERESs are then degraded by microautophagy, as suggested by the following observations. (i) Airyscan and CLEM microscopy show LC3 membranes intermixing with (expected in microautophagy) rather than encapsulating (expected in macroautophagy) Sec23 and procollagen. (ii) CLEM shows LC3 inside but not on the LAMP1-positive lysosome surface, as expected after lysosomal engulfment but not autophagosome–lysosome fusion. (iii) CLEM also shows encapsulation of LAMP1-positive membranes together with autophagic ERES inside the lysosome, as expected after lysosomal engulfment but not autophagosome–lysosome fusion. (iv) No effect of bafilomycin A1 on lysosomal internalization of autophagic ERESs containing procollagen is consistent with micro- but not macroautophagy. (v) Sec23 photobleaching experiments show rapid exchange of Sec23 between cytoplasm and LAMP1-positive autophagic ERESs, which is possible at ERESs partially engulfed by lysosomes in microautophagy but not after autophagosome–lysosome fusion.