AP1n

AP1n is a designation used in parts of the literature to refer to the mu subunit of the adaptor protein complex-1 (AP-1), a clathrin adaptor involved in vesicular trafficking. It is commonly called AP-1 mu1 and is encoded by the AP1M1 gene in humans. As part of the AP-1 complex, AP1n participates in forming a heterotetramer with the gamma-adaptin (AP1G1), beta-1 adaptin (AP1B1), and sigma-1 (AP1S1) subunits.

Functionally, AP1n contributes to cargo recognition and selection for transport at the trans-Golgi network and endosomes. The mu subunit provides primary binding to sorting signals on cargo proteins, particularly tyrosine-based YXXΦ motifs and dileucine motifs, guiding these proteins into clathrin-coated vesicles. The AP-1 coat then collaborates with clathrin and other factors to drive vesicle formation and trafficking to appropriate destinations within the cell.

Localization and interactions of AP1n reflect its role in membrane trafficking. AP-1 complexes associate with membranes through small GTPases and adapt other components of the coat, linking cargo selection to vesicle budding. The mu subunit's interactions with cargo signals and with other AP-1 subunits are essential for efficient endosome-to-TGN and TGN-to-endosome transport.

Genetically, AP1M1 encodes the mu1 subunit, and its expression is broadly distributed, consistent with a general role in cellular protein sorting. Disruption of AP-1 function can affect the trafficking of lysosomal enzymes and various membrane proteins, underscoring the importance of AP-1–mediated sorting in maintaining cellular homeostasis.