Gavin F. Painter

Identification of ARAP3, a novel PI3K effector regulating both Arf and Rho GTPases, by selective capture on phosphoinositide affinity matrices

We show that matrices carrying the tethered homologs of natural phosphoinositides can be used to capture and display multiple phosphoinositide binding proteins in cell and tissue extracts. We present the mass spectrometric identification of over 20 proteins isolated by this method, mostly from leukocyte extracts: they include known and novel proteins with established phosphoinositide binding domains and also known proteins with surprising and unusual phosphoinositide binding properties. One of the novel PtdIns(3,4,5)P3 binding proteins, ARAP3, has an unusual domain structure, including five predicted PH domains. We show that it is a specific PtdIns(3,4,5)P3/PtdIns(3,4)P2-stimulated Arf6 GAP both in vitro and in vivo, and both its Arf GAP and Rho GAP domains cooperate in mediating PI3K-dependent rearrangements in the cell cytoskeleton and cell shape.

The stress-activated phosphatidylinositol 3-phosphate 5-kinase Fab1p is essential for vacuole function in S. cerevisiae

Polyphosphoinositides have many roles in cell signalling and vesicle trafficking [1-3]. Phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2), a recently discovered PIP2 isomer, is ubiquitous in eukaryotic cells and rapidly accumulates in hyperosmotically stressed yeast. PI(3,5)P2 is synthesised from PI(3)P in both yeast and mammalian cells [4,5]. A search of the Saccharomyces cerevisiae genome database identified FAB1, a gene encoding a PIP kinase homologue and potential PI(3)P 5-kinase. Fab1p shows PI(3)P 5-kinase activity both in vivo and in vitro. A yeast strain in which FAB1 had been deleted was unable to synthesise PI(3,5)P2, either in the presence or absence of osmotic shock. A loss of PI(3,5)P2 was observed also in a temperature-sensitive FAB1 strain at the non-permissive temperature. A recombinant glutathione-S-transferase (GST)-Fab1p fusion protein was shown to have selective PI(3)P 5-kinase activity in vitro. Thus, we have demonstrated that Fab1p is a PI(3)P-specific 5-kinase and represents a third class of PIP kinase activity, which we have termed type III. Deletion of the FAB1 gene produces a loss of vacuolar morphology [6]; it is therefore concluded that PI(3,5)P2, the lipid product of Fab1p, is required for normal vacuolar function.

DAPP1 undergoes a PI 3-kinase-dependent cycle of plasma-membrane recruitment and endocytosis upon cell stimulation

BACKGROUNDnPhosphoinositide (PI) 3-kinase and its second messenger products, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) and phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2)), play important roles in signalling processes crucial for cell movement, differentiation and survival. Previously, we isolated a 32kDa PtdIns(3,4,5)P(3)-binding protein from porcine leukocytes. This protein contains an amino-terminal Src homology 2 (SH2) domain and a carboxy-terminal pleckstrin homology (PH) domain, and is identical to the recently described DAPP1 (also known as PHISH or Bam32) protein. Here, we characterised the subcellular distribution of DAPP1 in response to cell stimulation.nnnRESULTSnWhen expressed transiently in porcine aortic endothelial (PAE) cells, DAPP1 translocated from the cytosol to the plasma membrane in response to platelet-derived growth factor (PDGF). This translocation was dependent on both PI 3-kinase activity and an intact DAPP1 PH domain. Following recruitment to the plasma membrane, DAPP1 entered the cell in vesicles. Similar responses were seen in DT40 chicken B cells following antibody treatment, and Rat-1 fibroblasts following epidermal growth factor (EGF) or PDGF treatment. Colocalisation studies in PAE cells suggested entry of DAPP1 by endocytosis in a population of early endosomes containing internalised PDGF-beta receptors. DAPP1 also underwent PI 3-kinase-dependent phosphorylation on Tyr139 in response to PDGF stimulation, and this event was involved in the vesicular response.nnnCONCLUSIONSnThis is the first report of plasma-membrane recruitment and endocytosis of a PI 3-kinase effector protein in response to cell stimulation. The results suggest a novel role for DAPP1 in endosomal trafficking or sorting.

General synthesis of 3-phosphorylated myo-inositol phospholipids and derivatives

The D-3-phosphorylated myo-inositol phospholipids PtdIns(3)P, PtdIns(3,4)P2, PtdIns(3,4,5)P3 and PtdIns(3,5)P2 were synthesised from myo-inositol orthoformate 8. Key transformations included the regioselective DIBAL- and trimethylaluminium-mediated cleavages of myo-inositol orthoformate intermediates and a resolution–protection protocol using the camphor acetals 17. The final reductive debenzylation was effected with Pearlmans catalyst [Pd(OH)2] in the presence of sodium hydrogen carbonate. The biological properties of the phospholipids were evaluated against various protein kinases (PKB and PDK-1) in which they played an important activation role.

Synthesis and biological evaluation of aPtdIns(3,4,5)P3 affinity matrix

New PtdIns(3,4,5)P3 binding proteins have been nidentified utilising PtdIns(3,4,5)P3 modified affinity matrix 1 nwhich was synthesised from myo-inositol derivative 2, nphosphoramidite 9 and an agarose based solid support.

Synthesis of dipalmitoyl phosphatidylinositol 3,4-bis(phosphate)and 3,4,5-tris(phosphate) and their enantiomers

The dipalmitoyl derivatives 4 and 5 of 3-phosphorylated nmyo-inositol phospholipids 2 and 3 and their enantiomers are nsynthesised from homochiral myo-inositol precursors 6 and 11; nthey serve as biological probes for cell signal transduction.

VERSATILE SYNTHESIS OF MYO-INOSITOL PHOSPHOLIPID PRECURSORS

Homochiral myo-inositol derivatives 16 and 20 and their ncorresponding enantiomers possessing either the natural or unnatural ring nstereochemistry for inositol phospholipids are synthesised from nmyo-inositol derivatives 8 and 9 respectively using camphor ndimethyl acetals in a resolution–protection sequence.