Regulation of AP1 adaptor assembly by the bi-handed chaperone MEA1

Bidirectional trafficking between the trans-Golgi network (TGN) and endolysosomal compartments lies at the intersection of biosynthetic and degradative pathways. At the center of this trafficking route is the adaptor protein complex 1 (AP1), a heterotetramer essential for cargo recognition and vesicle budding. Here, we identified Male-Enhanced Antigen 1 (MEA1), a previously uncharacterized protein, as a critical AP1 regulator. Loss of MEA1 resulted in depletion of AP1 subunits and impaired trafficking of AP1-dependent cargoes. Mechanistically, MEA1 acts as a bi-handed chaperone, simultaneously engaging and stabilizing the μ1 and β1 subunits of AP1. The MEA1-stabilized μ1 and β1 collide with the γ and σ1 subunits stabilized by Alpha- and Gamma-Adaptin Binding Protein (AAGAB), another bi-handed chaperone, leading to formation of the tetrameric AP1 adaptor and release of both chaperones. These findings identify MEA1 as a key AP1 regulator and uncover a dual chaperone collision mechanism potentially generalizable to multiprotein complex assembly.