The function of AP1AR (adaptor related protein complex 1 associated regulatory protein, Ensembl gene identifier ENSG00000138660) is as follows. Necessary for adaptor protein complex 1 (AP-1)-dependent transport between the trans-Golgi network and endosomes. Regulates the membrane association of AP1G1/gamma1-adaptin, one of the subunits of the AP-1 adaptor complex. The direct interaction with AP1G1/gamma1- adaptin attenuates the release of the AP-1 complex from membranes. Regulates endosomal membrane traffic via association with AP-1 and KIF5B thus linking kinesin-based plus-end-directed microtubular transport to AP-1-dependent membrane traffic. May act as effector of AP-1 in calcium-induced endo-lysosome secretion. Inhibits Arp2/3 complex function; negatively regulates cell spreading, size and motility via intracellular sequestration of the Arp2/3 complex. {ECO:0000269|PubMed:15775984, ECO:0000269|PubMed:19706427, ECO:0000269|PubMed:21525240, ECO:0000269|PubMed:22689987}.