Crystal structure of a mammalian phosphoinositide-specific phospholipase C delta

Expression, purification, and kinetic characterization of a recombinant 80-kDa intracellular calcium-independent phospholipase A2

A CHO cell-derived 80-kDa recombinant polypeptide (GenBank number I15470I15470) putatively encoding a calcium-independent phospholipase A2 was expressed in S. frugiperda cells resulting in over a 15-fold increase in a calcium-independent phospholipase A1/A2 activity which was entirely inhibitable by (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one. The recombinant polypeptide was purified from cytosol by sequential tandem affinity chromatographies employing ATP-agarose and calmodulin-Sepharose stationary phases. This strategy resulted in the rapid purification (36 h) of recombinant phospholipase A2 activity in 56% overall yield to a single intense 80-kDa protein band on SDS-polyacrylamide gel electrophoresis after silver staining. The purified protein possessed phospholipase A1, phospholipase A2, and lysophospholipase activities. Microbore anion exchange chromatography demonstrated that the 80-kDa protein band was comprised of multiple distinct isoforms including an anionic isoform which possessed over a 5-fold higher specific activity (5 micromol/mg.min) than earlier eluting isoforms. Collectively, these results unambiguously demonstrate that: 1) the 80-kDa polypeptide catalyzes phospholipase A1/A2 and lysophospholipase activities with distinct kinetic parameters; 2) calmodulin and ATP both interact with the catalytic polypeptide independent of regulatory proteins; and 3) distinct isoforms of this polypeptide exist which possess markedly different specific activities.

Carboxyl-terminal fragments of phospholipase C-beta1 with intrinsic Gq GTPase-activating protein (GAP) activity

Fragments of the approximately 50 kDa COOH-terminal region of phospholipase C-beta1 (PLC-beta1(1)), ranging in size from 14 to 38 kDa, were expressed in Escherichia coli, purified, and tested for their regulatory activities. As expected, none of the fragments had phospholipase activity. Several fragments, referred to as PLC tails, displayed GTPase-activating protein (GAP) activity for Gq, the G protein class that stimulates the PLC-betas in response to receptors. Gq GAP activity is characteristic of intact PLC-betas. In reconstituted phospholipid vesicles that contained purified Gq and m1 muscarinic cholinergic receptors, the most active tails increased agonist-stimulated, steady-state GTPase activity over 4-fold. Stimulation of steady-state GTPase by the tails depended on receptors for facilitation of GDP-GTP exchange, suggesting that the tails act by accelerating hydrolysis of bound GTP. In addition to intrinsic GAP activity, one tail with high GAP activity and others with low or minimal activity potentiated the GAP activity of intact PLC-beta1. Other tails inhibited PLC-beta1s GAP effect. Both intrinsic GAP activity and potentiation of the PLC-beta1 GAP effect were often biphasic, with maxima as low as 100 nM tail and declining activities at higher concentrations. Several tails inhibited either the phospholipase activity of PLC-beta1, its stimulation by Gq, or both. The tails thus define the region of PLC-beta1 that has Gq GAP activity and suggest a mechanism of action in which the COOH terminus of PLC-betas can interact with Gq and with other PLC-beta1 molecules.

Myristoylated alanine-rich C kinase substrate (MARCKS) produces reversible inhibition of phospholipase C by sequestering phosphatidylinositol 4,5-bisphosphate in lateral domains

The myristoylated alanine-rich protein kinase C substrate (MARCKS) is a major protein kinase C (PKC) substrate in many different cell types. MARCKS is bound to the plasma membrane, and several recent studies suggest that this binding requires both hydrophobic insertion of its myristate chain into the bilayer and electrostatic interaction of its cluster of basic residues with acidic lipids. Phosphorylation of MARCKS by PKC introduces negative charges into the basic cluster, reducing its electrostatic interaction with acidic lipids and producing translocation of MARCKS from membrane to cytoplasm. The present study shows that physiological concentrations of MARCKS (<10 microM) inhibit phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) in phospholipid vesicles. A peptide corresponding to the basic cluster, MARCKS(151-175), produces a similar inhibition, which was observed with both PLC-delta1 and -beta1. Direct fluorescence microscopy observations demonstrate that the MARCKS peptide forms lateral domains enriched in the acidic lipids phosphatidylserine and PIP2 but not PLC, which accounts for the observed inhibition of PIP2 hydrolysis. Phosphorylation of MARCKS(151-175) by PKC releases the inhibition and allows PLC to produce a burst of inositol 1,4, 5-trisphosphate and diacylglycerol.

Annexins and protein kinases C

Annexins and protein kinases C belong to two distinct families of ubiquitous cytoplasmic proteins involved in signal transduction. All annexins share the property of binding calcium and phospholipids in the presence of calcium. Protein kinases C belong to three distinct groups of kinases: cPKCs (conventional PKCs) depend on calcium, diacylglycerol and negatively charged phospholipids for their activity, nPKCs (novel PKCs) depend on diacylglycerol and negatively charged phospholipids and aPKCs (atypical PKCs) only require negatively charged phospholipids. Almost all annexins are both in vitro and in vivo substrates for PKCs except annexin V. All annexins have a putative binding site for PKCs but only annexin V would possess a potential pseudo-substrate site. We propose that annexin V modulates the activity of some cPKCs on their substrates which may be the other annexins.

Positive charge at position 549 is essential for phosphatidylinositol 4,5-bisphosphate-hydrolyzing but not phosphatidylinositol-hydrolyzing activities of human phospholipase C delta1

Point mutagenesis, phosphatidylinositol (PI), and phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis assays and equilibrium centrifugation PIP2 assays were used to study the functional roles of four highly conserved arginine residues in the Y region of human phospholipase C delta1 (PLCdelta1) (Arg-527, -549, -556, -701). Most of the mutant enzymes were either partially defective or fully active in their abilities to catalyze the hydrolysis of PI or PIP2. However, upon substitution of Arg-549 by glycine or histidine, the mutant enzyme was defective in its ability to catalyze the hydrolysis of PIP2, but it is still able to hydrolyze PI. Replacing Arg-549 with lysine had little effect on the level of PI and PIP2 hydrolytic activities of the mutant enzyme. The residual PIP2 hydrolyzing activity of R549H is highly dependent on pH. R549H showed 5-10% of the PIP2-hydrolyzing activity of the native enzyme between pH 5 and 7 and nondetectable PIP2-hydrolyzing activity at pH 8. The PIP2-hydrolyzing activity of R549G was not detectable at all pH values. Kinetic analysis of PLCdelta1-catalyzed PIP2 hydrolysis revealed that the micellar dissociation constant Ks and interfacial Michaelis constant Km were similar in the native, R549K, and R549H enzymes; but the specific activity at the saturated substrate mole fraction and infinite level of substrate (Vmax) of the R549H mutant were reduced by a factor of 15. PIP2 competitively inhibits the native enzyme to hydrolyze PI at both pH 7 and 8. However, PIP2 inhibits R549H only at pH 7.0 and does not inhibit R549G at either pH. Taken together, these results suggest that positive charge at position 549 of PLCdelta1 protein is essential for the enzyme to recognize and catalyze the hydrolysis of PIP2 but not PI.

Platelet-derived growth factor-dependent activation of phosphatidylinositol 3-kinase is regulated by receptor binding of SH2-domain-containing proteins which influence Ras activity

Upon binding of platelet-derived growth factor (PDGF), the PDGF beta receptor (PDGFR) undergoes autophosphorylation on distinct tyrosine residues and binds several SH2-domain-containing signal relay enzymes, including phosphatidylinositol 3-kinase (PI3K), phospholipase C gamma (PLC gamma), the GTPase-activating protein of Ras (RasGAP), and the tyrosine phosphatase SHP-2. In this study, we have investigated whether PDGF-dependent PI3K activation is affected by the other proteins that associate with the PDGFR. We constructed and characterized a series of PDGFR mutants which contain binding sites for PI3K as well as one additional protein, either RasGAP, SHP-2, or PLC gamma. While all of the receptors had wild-type levels of PDGF-stimulated tyrosine kinase activity and associated with comparable amounts of PI3K activity, their abilities to trigger accumulation of PI3K products in vivo differed dramatically. The wild-type receptor, as well as receptors that recruited PI3K or PI3K and SHP-2, were all capable of fully activating PI3K. In contrast, receptors that associated with PI3K and RasGAP or PI3K and PLC gamma displayed a greatly reduced ability to stimulate production of PI3K products. When this series of receptors was tested for their ability to activate Ras, we observed a strong positive correlation between Ras activation and PI3K activation. Further investigation of the relationship between Ras and PI3K indicated that Ras was upstream of PI3K. Thus, activation of PI3K requires not only binding of PI3K to the tyrosine-phosphorylated PDGFR but accumulation of GTP-bound Ras as well. Furthermore, PLC gamma and RasGAP negatively modulate PDGF-dependent PI3K activation. Finally, PDGF-stimulated signal relay can be regulated by altering the ratio of SH2-domain-containing enzymes that are recruited to the PDGFR.

C2 domain conformational changes in phospholipase C-delta 1

The structure of the PH-domain truncated core of rat phosphoinositide-specific phospholipase C-delta 1 has been determined at 2.4 A resolution and compared to the structure previously determined in a different crystal form. The stereochemical relationship between the EF, catalytic, and C2 domains is essentially identical. The Ca2+ analogue Sm3+ binds at two sites between the jaws of the C2 domain. Sm3+ binding ejects three lysine residues which bridge the gap between the jaws and occupy the Ca2+ site in the apoenzyme, triggering a conformational change in the jaws. The distal sections of the C2 jaws move apart, opening the mouth by 9 A and creating a gap large enough to bind a phospholipid headgroup.