Phosphatidylinositol transfer proteins from higher eukaryotes

The orchestra of lipid-transfer proteins at the crossroads between metabolism and signaling

Within the eukaryotic cell, more than 1000 species of lipids define a series of membranes essential for cell function. Tightly controlled systems of lipid transport underlie the proper spatiotemporal distribution of membrane lipids, the coordination of spatially separated lipid metabolic pathways, and lipid signaling mediated by soluble proteins that may be localized some distance away from membranes. Alongside the well-established vesicular transport of lipids, non-vesicular transport mediated by a group of proteins referred to as lipid-transfer proteins (LTPs) is emerging as a key mechanism of lipid transport in a broad range of biological processes. More than a hundred LTPs exist in humans and these can be divided into at least ten protein families. LTPs are widely distributed in tissues, organelles and membrane contact sites (MCSs), as well as in the extracellular space. They all possess a soluble and globular domain that encapsulates a lipid monomer and they specifically bind and transport a wide range of lipids. Here, we present the most recent discoveries in the functions and physiological roles of LTPs, which have expanded the playground of lipids into the aqueous spaces of cells.

The lipid transfer activity of phosphatidylinositol transfer protein is sufficient to account for enhanced phospholipase C activity in turkey erythrocyte ghosts

BACKGROUND

The minor membrane phospholipid phosphatidylinositol 4, 5-bisphosphate (PIP2) has been implicated in the control of a number of cellular processes. Efficient synthesis of this lipid from phosphatidylinositol has been proposed to require the presence of a phosphatidylinositol/phosphatidylcholine transfer protein (PITP), which transfers phosphatidylinositol and phosphatidylcholine between membranes, but the mechanism by which PITP exerts its effects is currently unknown. The simplest hypothesis is that PITP replenishes agonist-sensitive pools of inositol lipids by transferring phosphatidylinositol from its site of synthesis to sites of consumption. Recent cellular studies, however, led to the proposal that PITP may play a more active role as a co-factor which stimulates the activity of phosphoinositide kinases and phospholipase C (PLC) by presenting protein-bound lipid substrates to these enzymes. We have exploited turkey erythrocyte membranes as a model system in which it has proved possible to distinguish between the above hypotheses of PITP function.

RESULTS

In turkey erythrocyte ghosts, agonist-stimulated PIP2 hydrolysis is initially rapid, but it declines and reaches a plateau when approximately 15% of the phosphatidylinositol has been consumed. PITP did not affect the initial rate of PIP2 hydrolysis, but greatly prolonged the linear phase of PLC activity until at least 70% of phosphatidylinositol was consumed. PITP did not enhance the initial rate of phosphatidylinositol 4-kinase activity but did increase the unstimulated steady-state levels of both phosphatidylinositol 4-phosphate and PIP2 by a catalytic mechanism, because the amount of polyphosphoinositides synthesized greatly exceeded the molar amount of PITP in the assay. Furthermore, when polyphosphoinositide synthesis was allowed to proceed in the presence of exogenous PITP, after washing ghosts to remove PITP before activation of PLC, enhanced inositol phosphate production was observed, whether or not PITP was present in the subsequent PLC assay.

CONCLUSION

PITP acts by catalytically transferring phosphatidylinositol down a chemical gradient which is created as a result of the depletion of phosphatidylinositol at its site of use by the concerted actions of the phosphoinositide kinases and PLC. PITP is therefore not a co-factor for the phosphoinositide-metabolizing enzymes present in turkey erythrocyte ghosts.

Phosphorylation and redistribution of the phosphatidylinositol-transfer protein in phorbol 12-myristate 13-acetate- and bombesin-stimulated Swiss mouse 3T3 fibroblasts

By immunofluorescence microscopy it was shown that the phosphatidylinositol-transfer protein (PI-TP) becomes associated with the Golgi membranes when confluent (quiescent) Swiss mouse 3T3 fibroblast cells are stimulated with phorbol 12-myristate 13-acetate (PMA) and bombesin. Dibutyryl cyclic AMP or dexamethasone had no effect on the intracellular redistribution of PI-TP. In exponentially growing cells and in serum-starved (semi-quiescent) cells, PI-TP is already associated with Golgi structures. Stimulation of semi-quiescent cells by PMA resulted in a rapid redistribution of PI-TP. A similar yet slower response was observed after stimulation with bombesin. Stimulation of semi-quiescent 3T3 cells by PMA significantly increased the phosphorylation of PI-TP, as shown by immunoprecipitation of PI-TP from pre-labelled cells. No significant increase in phosphorylation of PI-TP was observed after stimulation of these cells by bombesin. Purified PI-TP was shown to be a substrate for protein kinase C in vitro. The possibility that the phosphorylation of PI-TP after activation of protein kinase C is involved in the observed redistribution of PI-TP is discussed.