GFP-LC3 labels organised smooth endoplasmic reticulum membranes independently of autophagy

Disruption of autophagy leads to accumulation of intracellular multilamellar inclusions morphologically similar to organised smooth endoplasmic reticulum (OSER) membranes. However, the relation of these membranous compartments to autophagy is unknown. The purpose of this study was to test whether OSER plays a role in the autophagic protein degradation pathway. Here, GFP-LC3 is shown to localise to the OSER membranes induced by calnexin expression both in transiently transfected HEK293 cells and in mouse embryo fibroblasts. In contrast to GFP-LC3, endogenous LC3 is excluded from these membranes under normal conditions as well as after cell starvation. Furthermore, YFP-Atg5, a protein essential for autophagy and known to reside on autophagic membranes, is excluded from the calnexin-positive inclusion structures. In cells devoid of Atg5, a protein essential for autophagy and known to reside on autophagic membranes, colocalisation of calnexin with GFP-LC3 within the multilamellar bodies is preserved. I show that calnexin, a protein enriched in the OSER, is not subject to autophagic or lysosomal degradation. Finally, GFP-LC3 targeting to these membranes is independent of its processing and insensitive to drugs modulating autophagic and lysosomal protein degradation. These observations are inconsistent with a role of autophagic/lysosomal degradation in clearance of multilamellar bodies comprising OSER. Furthermore, GFP-LC3, a fusion protein widely used as a marker for autophagic vesicles and pre-autophagic compartments, may be trapped in this compartment and this artefact must be taken into account if the construct is used to visualise autophagic membranes.