Figure 1.

The exocytic Q/t-SNAREs Sso1/2 and Sec9 mediate the homotypic fusion of post-Golgi, Atg9-containing vesicles into Atg9 tubulovesicular clusters. This fusion step represents one of the earliest steps in autophagosome formation. Atg23 and Atg27 form a complex with Atg9, and affect its anterograde movement. It is therefore likely that TVCs contain Atg9, Atg23 and Atg27. Subsequent to TVC formation, one or more TVCs move from peripheral cellular sites to a perivacuolar site to form the phagophore assembly site. Additional SNARE-mediated fusion events may be required for the elongation of the TVCs to give rise to autophagosomes. The Atg9-homotypic fusion event may require a complex between Sso1, Sec9, and the v-SNAREs Sec22 or Ytk6. The endocytic Q/t-SNARE Tlg2 also forms a complex with Sso1 and Sec9, and affects Atg9 anterograde transport.