ARMH3 is an ARL5 effector that promotes PI4KB-catalyzed PI4P synthesis at the trans-Golgi network

ARL5 is a member of the ARF family of small GTPases that is recruited to the trans-Golgi network (TGN) by another ARF-family member, ARFRP1, in complex with the transmembrane protein SYS1. ARL5 recruits its effector, the multisubunit tethering complex GARP, to promote SNARE-dependent fusion of endosome-derived retrograde transport carriers with the TGN. To further investigate the function of ARL5, we sought to identify additional effectors. Using proximity biotinylation and protein interaction assays, we found that the armadillo-repeat protein ARMH3 (C10orf76) binds to active, but not inactive, ARL5, and that it is recruited to the TGN in a SYS1-ARFRP1-ARL5-dependent manner. Unlike GARP, ARMH3 is not required for the retrograde transport of various cargo proteins. Instead, ARMH3 functions to activate phosphatidylinositol 4-kinase IIIβ (PI4KB), accounting for the main pool of phosphatidylinositol 4-phosphate (PI4P) at the TGN. This function contributes to recruitment of the oncoprotein GOLPH3 and glycan modifications at the TGN. These studies thus identify the SYS1-ARFRP1-ARL5-ARMH3 axis as a regulator of PI4KB-dependent generation of PI4P at the TGN.