"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"

"Gel filtration, sedimentation velocity, and immunoprecipitation experiments revealed that beta4 is a component of a multisubunit complex (AP-4) that also contains the sigma4 polypeptide and two additional adaptor subunit homologs named mu4 (mu-ARP2) and epsilon."

"Immunogold electron microscopy indicates that AP-4 is associated with nonclathrin-coated vesicles in the region of the trans-Golgi network."

"The mu4 subunit of the complex specifically interacts with a tyrosine-based sorting signal, indicating that, like the other three AP complexes, AP-4 is involved in the recognition and sorting of cargo proteins with tyrosine-based motifs."

"Using a variety of protein interaction assays, we found that tepsin comprises two phylogenetically conserved peptide motifs, [GS]LFXG[ML]X[LV] and S[AV]F[SA]FLN, within its C-terminal unstructured region, which interact with the C-terminal ear (or appendage) domains of the β4 and ϵ subunits of AP-4, respectively."

"The heterotetrameric (ϵ-β4-μ4-σ4) complex adaptor protein 4 (AP-4) is a component of a non-clathrin coat involved in protein sorting at the trans-Golgi network (TGN)."

"We show that tepsin harbors a hydrophobic sequence, LFxG[M/L]x[L/V], in its unstructured C-terminus, which binds directly and specifically to the C-terminal β4 appendage domain."

"The adaptor protein 4 (AP4) complex (ϵ/β4/μ4/σ4 subunits) forms a non-clathrin coat on vesicles departing the trans-Golgi network."

"AP-4 mediates signal-dependent export of selected cargos--most prominently ATG9A--from the TGN to endosomal/peripheral compartments that support autophagosome formation."

"Loss of AP-4 causes ATG9A retention in the TGN and alters LC3B lipidation readouts, linking AP-4 to autophagy pathways."

"A landmark 2022 study by Davies et al. identified DAGLB as an AP-4 cargo protein with significant implications for neuronal development [davies-2022-daglb-endocannabinoid-abstract]. DAGLB is a serine lipase that hydrolyzes diacylglycerol (DAG) to generate 2-arachidonoylglycerol (2-AG), the most abundant endocannabinoid in the brain."

"Mattera et al. demonstrated that AAGAB directly binds to the ε and σ4 subunits of AP-4 and stabilizes both endogenous and recombinant AP-4 subunits by preventing their proteasomal degradation [mattera-2022-aagab-chaperone-abstract]."

"Recent work by Majumder et al. showed that AP-4 regulates the trafficking of Sortilin (SORT1) from the TGN to endo-lysosomes in neurons [majumder-2022-lysosome-abstract]."

"Proteomic studies identified these multipass transmembrane proteins as AP-4 cargoes [davies-2018-ap4-vesicles-abstract]. Both proteins colocalize with ATG9A and exhibit altered localization in AP-4-deficient cells."

"AP-4 recognizes a distinct type of tyrosine-based sorting signal with the consensus sequence YXXØE, where Ø represents a bulky hydrophobic residue [burgos-2010-app-sorting-abstract][mattera-2017-atg9a-export-abstract]."

"Tepsin depletion in cells results in partial accumulation of ATG9A at the TGN but causes distinct phenotypes compared to complete AP-4 depletion, indicating that tepsin has specialized functions beyond simply serving as a passive coat component, possibly participating in vesicle internalization or cargo delivery"