adaptorproteinkomplex

Adaptor protein complex, commonly abbreviated AP complex, refers to a family of heterotetrameric protein assemblies that mediate cargo selection and coat formation in clathrin-dependent vesicular trafficking. They function as molecular adaptors that connect transmembrane cargo proteins or cargo receptors to clathrin, promoting the assembly of clathrin-coated vesicles at specific cellular membranes.

AP complexes are generally composed of two large subunits, one medium (mu) subunit, and one small (sigma) subunit. The different members include AP-1, AP-2, AP-3 and AP-4, which associate with distinct membranes and trafficking routes, as well as AP-5, and in some organisms AP-6. In each complex, the mu subunit recognizes YXXΦ sorting motifs in cargo proteins, while the sigma subunit contributes to binding dileucine-based signals and stabilizing cargo interactions. The large subunits help determine membrane targeting and interact with clathrin and accessory factors.

Functionally, adaptor protein complexes select cargo, recruit clathrin to form a clathrin-coated pit or vesicle, and help drive vesicle scission and uncoating after delivery. They cycle between the cytosol and membranes in a regulated manner, often controlled by small GTPases such as ARF1, which promote membrane recruitment of the coat.

Pathways: AP-2 operates at the plasma membrane during clathrin-mediated endocytosis. AP-1 functions at the trans-Golgi network and endosomal membranes to sort cargo for delivery to endosomes, lysosomes, or the plasma membrane. AP-3 and AP-4 mediate trafficking to lysosome-related organelles and other post-Golgi destinations, whereas AP-5 is associated with endosomal compartments. The specific cargo and signals vary among complexes, but all rely on adaptor–cargo recognition and coat recruitment.

Clinical and research relevance: Mutations or dysregulation of AP complex subunits have been linked to human diseases, including neurodevelopmental disorders and metabolic defects, highlighting the role of adaptor complexes in cellular logistics and organismal health.