Figure 1.

Stages of vacuole fusion and formation of the vertex ring. (A) Vacuole fusion undergoes experimentally defined stages. 1. Priming–dispersed vacuoles harboring inactive cis-SNARE complexes are activated by the AAA+ ATPase Sec18 and its adaptor protein Sec17; 2. Tethering – association of vacuoles through the activity of Ypt7 and the HOPS complex; 3. Docking – Vacuoles become tightly apposed leading to the formation of the vertex ring (orange) and trans-SNARE paring; 5 – The docking to fusion transition may also go through a hemifusion intermediate where the outer leaflets mix while the inner leaflets remain intact and prevent content mixing. 6. – Fusion occurs at the vertex ring leading the merger of both membranes (green) and content mixing. Fusion also leads to the internalization of the boundary membrane (yellow and blue). (B) The vertex ring is enriched with the Q-SNAREs Vti1, Vam3, and Vam7, and the R-SNARE Nyv1. The vertex ring domain is also enriched with Ypt7, HOPS and regulatory lipids including PI3P, PI(4,5)P₂, DAG and ergosterol. (Adapted from reference 2).