The retromer complex and clathrin define an early endosomal retrograde exit site

Area of Science:

• Cell Biology
• Molecular Biology
• Membrane Trafficking

Background:

• Retrograde sorting from early endosomes to the trans Golgi network (TGN) involves clathrin, AP1, epsinR, and the retromer complex.
• The precise functional relationship between clathrin and retromer in this pathway remains incompletely understood.

Purpose of the Study:

• To investigate whether clathrin and retromer function on the same or parallel pathways in retrograde sorting.
• To elucidate the specific roles of clathrin and retromer in the transport of Shiga toxin B-subunit.

Main Methods:

• Ultrastructural colocalization studies of clathrin and the retromer subunit Vps26.
• Functional assays involving interference with Vps26 expression to assess retrograde transport.
• Analysis of Shiga toxin B-subunit and transferrin receptor localization under various experimental conditions (Vps26 depletion, clathrin depletion, low temperature).

Main Results:

• Clathrin and Vps26 colocalize on early/recycling endosomes containing Shiga toxin B-subunit.
• Inhibition of Vps26 impairs Shiga toxin B-subunit retrograde transport to the TGN, stabilizing endosomal tubules.
• Clathrin depletion leads to accumulation of Shiga toxin B-subunit and transferrin receptor near multivesicular endosomes, suggesting clathrin initiates sorting.

Conclusions:

• The retromer complex plays a crucial role in the retrograde transport of Shiga toxin B-subunit.
• Clathrin and retromer function in consecutive steps for retrograde sorting on early endosomes.
• Clathrin likely initiates retrograde sorting, with retromer subsequently processing the transport tubules.