Microtubules support production of starvation-induced autophagosomes but not their targeting and fusion with lysosomes.

Autophagy is a major catabolic pathway in eukaryotic cells whereby the lack of amino acids induces the formation of autophagosomes, double-bilayer membrane vesicles that mediate delivery of cytosolic proteins and organelles for lysosomal degradation. The biogenesis and turnover of autophagosomes in mammalian cells as well as the molecular mechanisms underlying induction of autophagy and trafficking of these vesicles are poorly understood. Here we utilized different autophagic markers to determine the involvement of microtubules in the autophagic process. We show that autophagosomes associate with microtubules and concentrate near the microtubule-organizing center. Moreover, we demonstrate that autophagosomes, but not phagophores, move along these tracks en route for degradation. Disruption of microtubules leads to a significant reduction in the number of mature autophagosomes but does not affect their life span or their fusion with lysosomes. We propose that microtubules serve to deliver only mature autophagosomes for degradation, thus providing a spatial barrier between phagophores and lysosomes.