Phosphatidylinositol-4-kinase type II alpha contains an AP-3-sorting motif and a kinase domain that are both required for endosome traffic

The adaptor complex 3 (AP-3) targets membrane proteins from endosomes to lysosomes, lysosome-related organelles and synaptic vesicles. Phosphatidylinositol-4-kinase type II a (PI4KII alpha) is one of several proteins possessing catalytic domains that regulate AP-3-dependent sorting. Here we present evidence that PI4KII alpha uniquely behaves both as a membrane protein cargo as well as an enzymatic regulator of adaptor function. In fact, AP-3 and PI4KII alpha form a complex that requires a dileucine-sorting motif present in PI4KII alpha. Mutagenesis of either the PI4KII alpha-sorting motif or its kinase-active site indicates that both are necessary to interact with AP-3 and properly localize PI4KII alpha to LAMP-1-positive endosomes. Similarly, both the kinase activity and the sorting signal present in PI4KII alpha are necessary to rescue endosomal PI4KII alpha siRNA-induced mutant phenotypes. We propose a mechanism whereby adaptors use canonical sorting motifs to selectively recruit a regulatory enzymatic activity to restricted membrane domains.