Identification and cDNA cloning of a novel mammalian C2 domain-containing phosphoinositide 3-kinase, HsC2-PI3K

Human phosphoinositide 3-kinase C2beta, the role of calcium and the C2 domain in enzyme activity

The cDNA for a human Class II phosphoinositide 3-kinase (PI 3-kinase C2beta) with a C2 domain was cloned from a U937 monocyte cDNA library and the enzyme expressed in mammalian and insect cells. Like other Class II PI 3-kinases in vitro, PI 3-kinase C2beta utilizes phosphatidylinositol (PI) and PI 4-monophosphate but not PI 4, 5-biphosphate as substrates in the presence of Mg2+. Remarkably, and unlike other PI 3-kinases, the enzyme can use either Mg-ATP or Ca-ATP to generate PI 3-monophosphate. PI 3-kinase C2beta, like the Class I PI 3-kinases, but unlike PI 3-kinase C2alpha, is sensitive to low nanomolar levels of the inhibitor wortmannin. The enzyme is not regulated by the small GTP-binding protein Ras. The C2 domain of the enzyme bound anionic phospholipids such as PI and phosphatidylserine in vitro, but did not co-operatively bind Ca2+ and phospholipids. Deletion of the C2 domain increased the lipid kinase activity suggesting that it functions as a negative regulator of the catalytic domain. Although presently it is not known whether PI 3-kinase C2beta is regulated by Ca2+ in vivo, our results suggest a novel role for Ca2+ ions in phosphate transfer reactions.

A type II phosphoinositide 3-kinase is stimulated via activated integrin in platelets. A source of phosphatidylinositol 3-phosphate

We have observed that aggregation of human platelets, caused by activation of integrin alphaIIb beta3 and its consequent binding of fibrinogen, stimulates a novel pathway for synthesis of phosphatidylinositol 3,4bisphosphate, thereby activating protein kinase B/Akt. Such synthesis depends upon both the generation of phosphatidylinositol 3-phosphate (PtdIns3P), which is sensitive to wortmannin (IC50 7 nM) and calpain inhibitors, and the phosphorylation of PtdIns3P by PtdIns3P 4-kinase. We now report that a recently characterized C2 domain-containing phosphoinositide 3-kinase isoform (HsC2-PI3K) is present in platelets and a leukemic cell line (CHRF-288) derived from megakaryoblasts, and is likely to be responsible for the stimulated synthesis of PtdIns3P observed in platelets. HsC2-PI3K, identifiable by Western blotting and immunoprecipitatable activity, is sensitive to wortmannin (IC50 6-10 nM), requires Mg2+, and shows strong preference for PtdIns over PtdIns4P or phosphatidylinositol 4,5-bisphosphate as substrate. HsC2-PI3K is activated severalfold when platelets aggregate in an alphaIIb beta3-dependent manner or when platelet or CHRF-288 lysates are incubated with Ca2+. Activation is prevented by calpain inhibitors. CHRF-288, which cannot undergo activation of alphaIIb beta3 and thereby aggregate in response to platelet agonists, do not generate PtdIns3P or activate HsC2-PI3K under conditions that stimulate other phosphoinositide 3-kinases. HsC2-PI3K may thus be an important effector for integrin-dependent signaling.

A novel class II phosphoinositide 3-kinase predominantly expressed in the liver and its enhanced expression during liver regeneration

We report herein the cloning and characterization of a novel class II phosphoinositide 3-kinase, termed PI3K-IIgamma, from the cDNA library of regenerating rat liver. This cDNA encodes a protein of 1505 amino acids with a calculated molecular mass of 170,972 Da. The amino acid sequence of PI3K-IIgamma is highly similar to those of class II PI 3-kinases, including murine Cpk-m/p170 and human HsC2-PI3K. It contains a C2 domain at the C terminus but no recognizable protein motifs at its N terminus. PI3K-IIgamma displays a restricted substrate specificity for PtdIns and PtdIns 4-P, but not for PtdIns 4,5-P2. By epitope tag immunocytochemistry, the immunoreactivity for PI3K-IIgamma is localized in the juxtanuclear Golgi region at high levels and also in the plasma and nuclear membranes at low levels. By Northern blot analysis and in situ hybridization histochemistry, PI3K-IIgamma mRNA expression is confined to the liver throughout the development with much higher expression in adult liver than in fetal liver. In addition, its expression increases during liver regeneration after partial hepatectomy with maximal expression after the growth period, suggesting that PI3K-IIgamma may function mainly in highly differentiated hepatic cells.

Cloning and characterization of a novel class II phosphoinositide 3-kinase containing C2 domain

Phosphoinositide 3-kinases (PI3Ks) have been shown to play critical roles in cell growth, differentiation, survival, and vesicular transport. Class II PI3Ks have been recently identified in mouse and human (PI3K-C2 alpha/m-p170/m-cpk and HsC2-PI3K) and in Drosophila (PI3K 68D/cpk) which contain C2 domain at the C-terminus. However, their physiological function is largely unknown. We report here cloning and characterization of murine PI3K-C2 gamma, a novel class II PI3K. The catalytic domain as well as C2 domain are highly conserved in the Class II PI3K family, while the N-terminal regions of these proteins share little similarity. Unlike other Class II PI3Ks, PI3K-C2 gamma exclusively expressed in the liver, and a N-terminal truncated form was found in lung and a certain hematopoietic cell line. Specific antiserum against PI3K-C2 gamma precipitated PI3K activity from the membrane fraction of mouse liver but not from heart. Recombinant PI3K-C2 gamma exhibited a restricted lipid substrate specificity; it phosphorylated phosphatidylinositol (PtdIns) and PtdIns4P but not PtdIns(4,5)P2. Deletion mutations revealed that both the N-terminal region and the C2 domain were critical for enzymatic activity. The murine PI3K-C2 gamma gene locus was mapped to the distal region of mouse chromosome 6 in a region of homology with human chromosome 12p, which is distinct from the position of HsC2-PI3K. Cloning and biochemical characterization of the third member of class II PI3Ks provide a new insight into the function of this subfamily of PI3Ks.