AP-4, a novel protein complex related to clathrin adaptors

Abstract

Here we report the identification and characterization of AP-4, a novel protein complex related to the heterotetrameric AP-1, AP-2, and AP-3 adaptors that mediate protein sorting in the endocytic and late secretory pathways. The key to the identification of this complex was the cloning and sequencing of two widely expressed, mammalian cDNAs encoding new homologs of the adaptor β and σ subunits named β4 and σ4, respectively. An antibody to β4 recognized in human cells an ~83-kDa polypeptide that exists in both soluble and membrane-associated forms. Gel filtration, sedimentation velocity, and immunoprecipitation experiments revealed that β4 is a component of a multisubunit complex (AP-4) that also contains the σ4 polypeptide and two additional adaptor subunit homologs named μ4 (μ-ARP2) and ε. Immunofluorescence analyses showed that AP-4 is associated with the trans- Golgi network or an adjacent structure and that this association is sensitive to the drug brefeldin A. We propose that, like the related AP-1, AP-2, and AP-3 complexes, AP-4 plays a role in signal-mediated trafficking of integral membrane proteins in mammalian cells.