Figure 10.

Model Illustrating MVB Maturation from the TGN.

According to this model, the TGN is continually formed and released from the Golgi stack. It also functions as an EE and receives incoming cargo from the PM via CCVs. As it differentiates, the TGN probably subdivides into domains where SVs are released to the PM, into domains releasing CCVs for recycling to the PM (recycling endosomes) and into a domain that matures into an MVB. Participating in the latter process, as indicated, are the ESCRT complexes I, II, and III, as well as annexin. As in mammalian cells, we postulate that post-TGN trafficking of soluble proteins to the lytic compartment (vacuole) occurs receptor independently and is accompanied by a gradual transformation of parts of the EE (TGN) into the LE (MVB), which ultimately fuses with the vacuole membrane.