Autophagy or the degradation of cytosolic components in lysosomes is a pathway utilized by all cell types as a means to overcome starvation, recycle nutrients and remove unwanted or damaged intracellular constituents including both proteins and organelles (Mizushima et al., 2008
). In addition, delivery of autophagic cargo to late endosomes for complete or partial degradation has been described (Dengjel et al., 2005
; Nimmerjahn et al., 2003
). To date, three autophagy-related pathways have been described in higher eukaryotes: macroautophagy (MA), chaperone-mediated autophagy (CMA) and microautophagy (Cuervo, 2010
; Mizushima et al., 2008
). In MA, whole cytosolic regions are sequestered inside a vesicle (autophagosome) that then fuses with lysosomes or with late endosomal multivesicular bodies (MVB) (Mizushima et al., 2008
). CMA is a more selective autophagy that relies on the recognition by the hsc70 chaperone of amino acid motifs on cytosolic substrate proteins biochemically related to the pentapeptide KFERQ (Chiang et al., 1989
; Dice, 1990
). The complex hsc70/ cytosolic substrate binds to the lysosome-associated membrane protein type 2A (LAMP-2A) (Cuervo and Dice, 1996
) and after unfolding substrate proteins are translocated into lysosomes for degradation assisted by a luminal form of hsc70 (Cuervo, 2010
). A third autophagic pathway, microautophagy, has been described in yeast but has not yet been well characterized in eukaryotic cells (Marzella et al., 1981
). This pathway involves internalization of cytosolic cargo through invaginations of the lysosomal membrane (Marzella et al., 1981
), which resemble the formation of multivesicular bodies (MVB). The molecular mechanisms that mediate microautophagy-like delivery of cytosolic cargo to lysosomes in mammals remain unknown. The relationship between the invagination of the membrane during this process and MVB biogenesis is also not clear.
In this report, we have analyzed the autophagic mechanisms operating in late endosomal MVB comparatively to those active in lysosomes. Using a series of mutants, we present evidence that MA is operative in LE, whereas LAMP-2A-mediated CMA is not involved in cytosolic protein translocation in the MVB lumen or in the luminal vesicles. Instead, we provide evidence that a microautophagy-like process occurs during MVB biogenesis to deliver selected soluble cytosolic proteins to the vesicles of late endosomes/MVB. This process is mediated by chaperones, but in contrast to CMA that occurs in lysosomes, endosomal microautophagy: 1) relies on the ESCRT I and III complexes, required for the formation of the vesicles in which the cytosolic cargo is internalized, and 2) relies on protein cargo delivery by hsc70 through electrostatic interactions of this chaperone with the endosomal limiting membrane. Therefore, we propose that endosomal microautophagy shares molecular components with both the endocytic and autophagic pathways and contributes to degradation of the soluble cytosol.