18 Jan 2023
by Richa

Summary- paper 7: The yeast endocytic early/sorting compartment exists as an independent sub-compartment within the trans-Golgi network

Junko Y. Toshima, Ayana Tsukahara, Makoto Nagano, Takuro Tojima, Daria E. Siekhaus, Akihiko Nakano, Jiro Toshima

Biorxiv, 2022

Questions/gaps addressed:

  • How and where do uncoated endocytic vesicles get delivered to the early/sorting endosome compartment in yeast? i.e. What is the first destination of endocytic cargos in yeast? Which Q-SNARE (target SNARE) engages with the vesicles (Tlg2?)?

  • Conflicting observations in literature- (1) Sec7-residing TGN (trans-Golgi network) is the first endocytic compartment; (2) two endosomal compartments in yeast- one with Rab5/Vps21, other with Rab7/Ypt7; (3) endocytosed cargos rarely transported to Sec7-TGN. Why the inconsistencies? Limitation in imaging technologies?

Key methods:

  • Simultaneous triple-color and 4D (3D plus time) super-resolution confocal live imaging microscopy (SCLIM) to monitor cargo trafficking in yeast

Major takeaways:

  • Localization dynamics of Tlg2 and Sec7 at the TGN are distinct. Tlg2 appears at the TGN earlier than Sec7 and disappears before Sec7 from the TGN.

  • Localization of pHrode Red-α-factor (endocytic cargo) overlaps more with Tlg2 5 min post endocytosis than Sec7. Tlg2 appears to localize with the cargo throughout the early-to-late stage endosomes.

  • Vps21p partially localizes around the TGN (overlap with Sec7) as well as in endosomal compartments (overlap with Tlg2). Tlg2 is transported to the PVC (prevacuolar compartment) via the Vps21-residing compartment without passing through the Sec7-TGN. How does Tlg2 get back to the early-TGN?

  • γ-adaptin ear containing, Arf binding protein (GGA) adaptors, Gga1 and Gga2; and epsin-related proteins, Ent3/5 are required for delivery of α-factor from TGN to PVC. AP1 adaptor not needed. GGAs arrive at the TGN earlier than AP1.

  • Gga1/2 important for delivery of Tlg2 compartment to endosome (signal for Gga- Ub on cargos destined for lysosomal degradation?). Ent3/5 important for Vps9 (GEF for Vps21) delivery from TGN to endosome. Nice model figure!