Figure 1

Schematic model of the different types of autophagy in mammalian cells. Three different main types of autophagy have been described in mammalian cells. In macroautophagy, entire regions of the cytosol, including proteins and organelles, are sequestered in a double membrane vesicle that then fuses with lysosomes to enable cargo degradation. During microautophagy, the lysosomal membrane invaginates or protrudes to internalize cytosolic components in single membrane vesicles that are then rapidly degraded in the lysosomal lumen. Selective degradation of soluble cytosolic proteins can be attained by chaperone-mediated autophagy, in which candidate substrate proteins (green) are brought to a translocation complex (yellow) at the lysosomal membrane by cytosolic chaperones (red). Substrate proteins can only reach the lysosomal lumen by this pathway after undergoing complete unfolding in the cytosolic side of the lysosomal membrane. The two most important functions of autophagy are also depicted in this model: (i) as an alternative source of energy, by providing essential components - such as amino acids (aa) or free fatty acids (FA) - that can be used for cellular fueling; and (ii) as an essential component of the cellular mechanisms for quality control, by guaranteeing removal of altered proteins and organelles.