Recently, on the basis of the machinery for uptake of microautophagic cargoes into lysosomes and late endosomes (endolysosomes), two different types of microautophagy have been proposed20: (1) fission-type microautophagy and (2) fusion-type microautophagy. In macroautophagy, lysosomes are engaged at the end of the autophagic process by heterotypic fusion with an autophagosome or an amphisome (the product of a late endosome or multivesicular body (MVB) fusing with an autophagosome)21,22. Fusion-type microautophagy, like macroautophagy, requires the core autophagy machinery and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. By contrast, fission-type microautophagy is independent of the core autophagy machinery and is mediated by endosomal sorting complexes required for transport (ESCRTs) that mediate membrane scission or budding. In mammalian cells, most microautophagic processes studied so far belong to the fission type, but the exact molecular mechanisms and biological functions remain to be fully elucidated.
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