Rab4b controls an early endosome sorting event by interacting with the γ-subunit of the clathrin adaptor complex 1

The endocytic pathway is essential for cell homeostasis and numerous small Rab GTPases are involved in its control. The endocytic trafficking step controlled by Rab4b has not been elucidated, although recent data suggested it could be important for glucose homeostasis, synaptic homeostasis or adaptive immunity. Here, we show that Rab4b is required for early endosome sorting of transferrin receptors (TfRs) to the recycling endosomes, and we identified the AP1γ subunit of the clathrin adaptor AP-1 as a Rab4b effector and key component of the machinery of early endosome sorting. We show that internalised transferrin (Tf) does not reach Vamp3/Rab11 recycling endosomes in the absence of Rab4b, whereas it is rapidly recycled back to the plasma membrane. By contrast, overexpression of Rab4b leads to the accumulation of internalised Tf within AP-1- and clathrin-coated vesicles. These vesicles are poor in early and recycling endocytic markers except for TfR and require AP1γ for their formation. Furthermore, the targeted overexpression of the Rab4b-binding domain of AP1γ to early endosome upon its fusion with FYVE domains inhibited the interaction between Rab4b and endogenous AP1γ, and perturbed Tf traffic. We thus proposed that the interaction between early endocytic Rab4b and AP1γ could allow the budding of clathrin-coated vesicles for subsequent traffic to recycling endosomes. The data also uncover a novel type of endosomes, characterised by low abundance of either early or recycling endocytic markers, which could potentially be generated in cell types that naturally express high level of Rab4b.