Involvement of Atg9-containing structures in the membrane dynamics of autophagy. Atg9 vesicles are derived from the Golgi apparatus as single-membrane structures with a diameter of 30–60 nm. During starvation, a small number of Atg9 vesicles assemble to the PAS to become part of the isolation membrane and ultimately part of the autophagosomal outer membrane. After autophagosome formation, Atg9 clusters remaining on the outer membrane are recycled back to the cytoplasm: (i) Atg9 vesicle recycling is coupled with fusion of the autophagosomal outer membrane with the vacuolar membrane; or (ii) the Atg9 vesicle is recycled from the vacuolar membrane (alternatively, Atg9 is translocated to the endosome or the Golgi apparatus by retrograde transport and generated via the Golgi-related secretory system as well as the biogenesis of Atg9 vesicles). The asymmetrical distribution of Atg9 on the outer membrane would allow Atg9 to avoid degradation and to be recycled back to the cytoplasm. IM and OM, autophagosomal inner and outer membranes, respectively.