Regulation of phosphoinositide-metabolizing enzymes by clathrin coat proteins

Interaction of Fapp1 with Arf1 and PI4P at a membrane surface: an example of coincidence detection

Interactions among ADP-ribosylation factors (ARFs), various adaptor proteins, and membrane lipids are essential for intracellular vesicle transport of a variety of cellular materials. Here, we present nuclear magnetic resonance (NMR)-based information on the nature of the interaction of yeast Arf1 (yArf1) and the pleckstrin homology (PH) domain of four-phosphate-adaptor protein 1 (Fapp1) as it occurs at a model membrane surface. Interactions favor a model in which Fapp1 is partially embedded in the membrane and interacts with a membrane-associated Arf1 molecule primarily through contacts between residues in switch I of Arf1 and regions near and under the solution exposed C-terminal extension of the PH domain. The Arf1 binding site on Fapp1-PH is distinct from a positively charged phosphatidylinositol-4-phosphate (PI4P) binding site. A structural model is constructed that supports coincidence detection of both activated ARF and PI4P as a mechanism facilitating Fapp1 recruitment to membranes.

Spatiotemporal control of endocytosis by phosphatidylinositol-3,4-bisphosphate

Phosphoinositides serve crucial roles in cell physiology, ranging from cell signalling to membrane traffic. Among the seven eukaryotic phosphoinositides the best studied species is phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2), which is concentrated at the plasma membrane where, among other functions, it is required for the nucleation of endocytic clathrin-coated pits. No phosphatidylinositol other than PI(4,5)P2 has been implicated in clathrin-mediated endocytosis, whereas the subsequent endosomal stages of the endocytic pathway are dominated by phosphatidylinositol-3-phosphates(PI(3)P). How phosphatidylinositol conversion from PI(4,5)P2-positive endocytic intermediates to PI(3)P-containing endosomes is achieved is unclear. Here we show that formation of phosphatidylinositol-3,4-bisphosphate (PI(3,4)P2) by class II phosphatidylinositol-3-kinase C2α (PI(3)K C2α) spatiotemporally controls clathrin-mediated endocytosis. Depletion of PI(3,4)P2 or PI(3)K C2α impairs the maturation of late-stage clathrin-coated pits before fission. Timed formation of PI(3,4)P2 by PI(3)K C2α is required for selective enrichment of the BAR domain protein SNX9 at late-stage endocytic intermediates. These findings provide a mechanistic framework for the role of PI(3,4)P2 in endocytosis and unravel a novel discrete function of PI(3,4)P2 in a central cell physiological process.

The lipid kinase PI4KIIIβ preserves lysosomal identity

Lipid modifications are essential in cellular sorting and trafficking inside cells. The role of phosphoinositides in trafficking between Golgi and endocytic/lysosomal compartments has been extensively explored and the kinases responsible for these lipid changes have been identified. In contrast, the mechanisms that mediate exit and recycling from lysosomes (Lys), considered for a long time as terminal compartments, are less understood. In this work, we identify a dynamic association of the lipid kinase PI4KIIIβ with Lys and unveil its regulatory function in lysosomal export and retrieval. We have found that absence of PI4KIIIβ leads to abnormal formation of tubular structures from the lysosomal surface and loss of lysosomal constituents through these tubules. We demonstrate that the kinase activity of PI4KIIIβ is necessary to prevent this unwanted lysosomal efflux under normal conditions, and to facilitate proper sorting when recycling of lysosomal material is needed, such as in the physiological context of lysosomal reformation after prolonged starvation.

Phosphatidylinositol-4-phosphate 5-kinase isoforms exhibit acyl chain selectivity for both substrate and lipid activator

Phosphatidylinositol 4,5-bisphosphate is mostly produced in the cell by phosphatidylinositol-4-phosphate 5-kinases (PIP5K) and has a crucial role in numerous signaling events. Here we demonstrate that in vitro all three isoforms of PIP5K, α, β, and γ, discriminate among substrates with different acyl chains for both the substrates phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol (PtdIns) although to different extents, with isoform γ being the most selective. Fully saturated dipalmitoyl-PtdIns4P was a poor substrate for all three isoforms, but both the 1-stearoyl-2-arachidonoyl and the 1-stearoyl-2-oleoyl forms of PtdIns4P were good substrates. V(max) was greater for the 1-stearoyl-2-arachidonoyl form compared with the 1-stearoyl-2-oleoyl form, although for PIP5Kβ the difference was small. For the α and γ isoforms, K(m) was much lower for 1-stearoyl-2-oleoyl PtdIns4P, making this lipid the better substrate of the two under most conditions. Activation of PIP5K by phosphatidic acid is also acyl chain-dependent. Species of phosphatidic acid with two unsaturated acyl chains are much better activators of PIP5K than those containing one saturated and one unsaturated acyl chain. PtdIns is a poor substrate for PIP5K, but it also shows acyl chain selectivity. Curiously, there is no acyl chain discrimination among species of phosphatidic acid in the activation of the phosphorylation of PtdIns. Together, our findings indicate that PIP5K isoforms α, β, and γ act selectively on substrates and activators with different acyl chains. This could be a tightly regulated mechanism of producing physiologically active unsaturated phosphatidylinositol 4,5-bisphosphate species in the cell.