When the proteasome is not enough: autophagy | Visual AI Learning Guide

Watch the membrane grow. A phagophore, a double-membrane cup, nucleates in the cytoplasm and expands around a patch of cargo — a protein aggregate or a damaged organelle. When the edges fuse, you have a sealed autophagosome. That vesicle then docks with a lysosome, the membranes merge, and lysosomal hydrolases digest the contents into amino acids and small peptides that are recycled. That is macroautophagy. The second route, chaperone-mediated autophagy, works differently. A chaperone recognizes a KFERQ-like motif on a substrate, delivers it to LAMP2A at the lysosomal membrane, and the chain is unfolded and threaded directly into the lysosome. No vesicle is built. The key distinction is cargo size and solubility: the proteasome handles individual soluble proteins, while autophagy handles what the proteasome cannot — large aggregates, oligomers, and whole organelles.

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The proteasome can only degrade unfolded, soluble polypeptides that fit through its narrow channel. Large aggregates, oligomers, and damaged organelles are too big, so the cell routes them to the lysosome through autophagy. In macroautophagy, a double-membrane phagophore expands and seals into an autophagosome that engulfs cytoplasmic cargo. The autophagosome fuses with a lysosome, and lysosomal hydrolases degrade the contents. In chaperone-mediated autophagy, a chaperone delivers a substrate bearing a KFERQ-like motif directly to the lysosomal membrane protein LAMP2A, which translocates the unfolded chain into the lysosome. Both routes converge on lysosomal degradation, and both are induced when the load of misfolded protein or damaged organelles rises. The distinction matters because the two systems handle different cargo: the proteasome processes individual soluble proteins, while autophagy clears bulk aggregates and organelles that the proteasome cannot physically accommodate.