Rabaptin-5alpha/rabaptin-4 serves as a linker between rab4 and gamma1-adaptin in membrane recycling from endosomes

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Abstract
Rab4 regulates recycling from early endosomes. We investigated the role of the rab4 effector rabaptin‐5α and its putative partner γ1‐adaptin in membrane recycling. We found that rabaptin‐5α forms a ternary complex with the γ1–σ1 subcomplex of AP‐1, via a direct interaction with the γ1‐subunit. The binding site for γ1‐adaptin is in the hinge region of rabaptin‐5α, which is distinct from rab4‐ and rab5‐binding domains. Endogenous or ectopically expressed γ1‐ adaptin localized to both the trans‐Golgi network and endosomes. Co‐expressed rabaptin‐5α and γ1‐adaptin, however, co‐localized in a rab4‐dependent manner on recycling endosomes. Transfection of rabaptin‐5α caused enlarged endosomes and delayed recycling of transferrin. RNAi of rab4 had an opposing effect on transferrin recycling. Collectively, our data show that rab4‐GTP acts as a scaffold for a rabaptin‐5α–γ1‐adaptin complex on recycling endosomes and that interactions between rab4, rabaptin‐5α and γ1‐adaptin regulate membrane recycling.