Activation of the phospholipase D (PLD) pathway is a widespread response when cells are activated by agonists that bind receptors on the cell surface. A 16-kD cytosolic component can reconstitute guanosine triphosphate (GTP)-mediated activation of phospholipase D in HL60 cells depleted of their cytosol by permeabilization. This factor was purified and identified as two small GTP-binding proteins, ARF1 and ARF3. Recombinant ARF1 substituted for purified ARF proteins in the reconstitution assay. These results indicate that phospholipase D is a downstream effector of ARF1 and ARF3. The well-established role of ARF in vesicular traffic would suggest that alterations in lipid content by PLD are an important determinant in vesicular dynamics.

Interaction of ligands with 'Ca2+-mobilizing' receptors is thought to result in the generation of two second messengers, inositol trisphosphate and diacylglycerol, from a common substrate, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) (refs 1, 2), a component of plasma membranes. It is not known how the occupation of such receptors is translated into the activation of the catalytic unit polyphosphoinositide (PPI) phosphodiesterase, and then to cellular activation, but our recent experiments suggest that GTP regulatory proteins may be involved. In mast cells, non-hydrolysable analogues of GTP introduced and then trapped in the cytosol are able to substitute for external ligands in inducing exocytosis, a well-defined Ca2+-dependent process, suggesting that guanine nucleotide regulatory proteins may act by stimulating the catalytic activity of the PPI phosphodiesterase. We now provide evidence that mast cell secretion is inhibited by internalized neomycin, a compound known to interact with PPI. We also show that the PPI phosphodiesterase of human neutrophil plasma membranes can be activated simply by adding GTP analogues in the presence of concentrations of Ca2+ that pertain in unstimulated cells. These findings strongly support the idea that the coupling factor linking receptor and PPI phosphodiesterase is a guanine nucleotide binding protein analogous to those involved in the activation and inhibition of adenylate cyclase.

Sterol traffic between the endoplasmic reticulum (ER) and plasma membrane (PM) is a fundamental cellular process that occurs by a poorly understood non-vesicular mechanism. We identified a novel, evolutionarily diverse family of ER membrane proteins with StART-like lipid transfer domains and studied them in yeast. StART-like domains from Ysp2p and its paralog Lam4p specifically bind sterols, and Ysp2p, Lam4p and their homologs Ysp1p and Sip3p target punctate ER-PM contact sites distinct from those occupied by known ER-PM tethers. The activity of Ysp2p, reflected in amphotericin-sensitivity assays, requires its second StART-like domain to be positioned so that it can reach across ER-PM contacts. Absence of Ysp2p, Ysp1p or Sip3p reduces the rate at which exogenously supplied sterols traffic from the PM to the ER. Our data suggest that these StART-like proteins act in trans to mediate a step in sterol exchange between the PM and ER.DOI: http://dx.doi.org/10.7554/eLife.07253.001

The X-linked disorder oculocerebrorenal syndrome of Lowe is caused by mutation of the OCRL1 protein, an inositol polyphosphate 5-phosphatase. OCRL1 is localised to the Golgi apparatus and early endosomes, and can translocate to lamellipodia upon growth factor stimulation. We show here that OCRL1 interacts with several members of the rab family of small GTPases. Strongest interaction is seen with Golgi-associated rab1 and rab6 and endosomal rab5. Point mutants defective in rab binding fail to target to the Golgi apparatus and endosomes, strongly suggesting rab interaction is required for targeting of OCRL1 to these compartments. Membrane recruitment via rab binding is required for changes in Golgi and endosomal dynamics induced by overexpression of catalytically inactive OCRL1. In vitro experiments demonstrate that rab5 and rab6 directly stimulate the 5-phosphatase activity of OCRL1. We conclude that rabs play a dual role in regulation of OCRL1, firstly targeting it to the Golgi apparatus and endosomes, and secondly, directly stimulating the 5-phosphatase activity of OCRL1 after membrane recruitment.

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