Reconsidering the selectivity of bulk autophagy: cargo hitchhiking specifies cargo for degradation

Bulk macroautophagy/autophagy, typically induced by starvation, is generally thought to isolate cytosolic components for degradation in a non-selective manner. Despite the fundamental nature of the eukaryotic degradation pathway, the question of what cargo is isolated by autophagy has remained unaddressed for over 30 years. We recently employed mass spectrometry to analyze the contents of isolated autophagic bodies. In the process of these experiments, we uncovered Hab1 (Highly enriched in Autophagic Bodies 1), a novel protein that is delivered extremely preferentially via autophagy. We report that Hab1 is a novel receptor protein, the N-terminus of which binds Atg8-PE, whereas the C-terminus binds ribosomes. Surprisingly, detailed biochemical and microscopic analyses revealed that ribosome-bound Hab1 is preferentially delivered to the vacuole by "'hitchhiking'" on phagophores/isolation membranes that form during bulk autophagy. This is a completely different mechanism of cargo selection that differs from previous descriptions of selective autophagy, in which the cargo-specific receptor proteins initiate phagophore membrane formation via scaffold proteins such as Atg11. We propose that cargo hitchhiking allows for the specification of cargo during bulk autophagy, which is otherwise a non-selective process.