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

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.