Mammalian cells express two classes of phosphatidylinositol 4-kinase (PI4K), designated as Types II and III, that phosphorylate phosphatidylinositol to generate PI4P. A number of studies have indicated that these enzymes are important for Golgi trafficking and both early and late stages of endocytosis. In this study, we focus on PI4KIIβ, a protein that is evenly distributed between membrane and soluble fractions, and is believed to participate in stimulus-dependent phosphoinositide signaling. Using molecular brightness analysis, we found that EGFP-tagged PI4KIIβ exists as two distinct species in the cytoplasm: a soluble monomer and a high-order complex enriched with multiple copies of PI4KIIβ. This observation was confirmed by an autocorrelation analysis that identified two species with distinct mobilities. We further demonstrate that the high-order complex enriched with PI4KIIβ is sensitive to inhibition of palmitoylation, indicating that it is associated with membranes, very likely vesicles. Indeed, we show that the high-order PI4KIIβ complex is sensitive to expression of dynamin 2 (K44A), a dominant-negative inhibitor of endocytosis. Using dual-color heterospecies partition analysis, we directly detected that PI4KIIβ comoves with clathrin light chain on vesicles. This analysis allows us to isolate the comobile species in the presence of strong background contribution from the monomeric pool of PI4KIIβ. Our results strongly suggest that PI4KIIβ is involved in an early stage of endocytosis and is associated with clathrin-coated vesicles. Moreover, we establish molecular brightness as a powerful tool for characterizing cellular cytosolic vesicles that are otherwise difficult to characterize by other techniques.
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