The autophagosome: current understanding of formation and maturation

Autophagy is an important and highly conserved catabolic process with roles in development, homeostasis, and cellular stress responses. It describes various distinct pathways for the delivery of cytoplasmic materials (including misfolded protein aggregates and some organelles) to the lysosome for degradation and component recycling. The best understood form of autophagy (macroautophagy) describes the de novo assembly, maturation, and trafficking of a unique double membrane-bound organelle - the autophagosomes - that sequesters cytoplasmic materials and ultimately merges with the lysosomal compartment to form a degradative autolysosome. To rapidly assemble such a structure in response to stimuli, cells express a family of dedicated autophagy-related (ATG) gene products that act sequentially to control membrane events leading first to the nucleation of an isolation membrane or phagophore, followed by phagophore expansion, and sealing to form an autophagosome that traffics to - and ultimately fuses with - the lysosome. These molecules are activated in response to upstream signaling pathways (notably, the mechanistic target of rapamycin [mTOR] pathway), and comprise protein and lipid kinases, putative membrane coats, and unique ubiquitin-like conjugation systems. In concert, a barrage of accessory proteins involved in various membrane trafficking pathways focused on the endosomal compartment are co-opted at the assembly site to facilitate autophagosome biogenesis. Understanding the integrated pathways that coordinate autophagosome assembly at the molecular level will be crucial if we are to realize the potential for autophagy manipulation in future disease therapies.