Phosphoinositide Kinases

The membranes of all metazoan cells contain a substantial amount of phosphatidylinositol (PtdIns) and at least seven phosphorylated PtdIns derivatives (PPIn) at 30- to 1000-fold lower concentrations. Phosphoinositide kinases synthesize the various PPIn by adding phosphate groups to pre-existing inositol glycerophospholipids.

The kinase domains of the two PtdIns 4-kinase subfamilies, the various subfamilies of phosphoinositide 3-kinase (PI3K) and protein kinases are closely related. By contrast, the three phosphatidylinositol phosphate kinases (PIPkins) constitute a unique kinase family (though sharing some kinase domain motifs with other kinases), with each donating a phosphate to only one of the three isomeric PtdInsPs.

PtdIns(4,5)P₂ is a key phosphoinositide. It is the shared substrate of two widespread receptor-activated signaling pathways – the phospholipase C-catalyzed formation of Ins(1,4,5)P₃ and 1,2-diacylglycerol (DAG), and synthesis of the membrane-associated messenger molecule PtdIns(3,4,5)P₃ by Type I PI3Ks – and is also essential for a substantial number of other cell functions, including actin-based motility, secretory vesicle trafficking, exocytosis and phospholipase D activation.

Four kinases contribute to PtdIns(4,5)P₂ biosynthesis. Most is made, via PtdIns4P, by a PI4K and PIPkin-1 at the plasma membrane, secretory vesicles, Golgi and nuclei (and probably elsewhere). Cells also use another pathway, involving an uncharacterized PtdIns 5-kinase and PIPkin-II, to make a small proportion of cellular PtdIns(4,5)P₂ via PtdIns5P,. Reasons for the coexistence of these two pathways remain uncertain. There is evidence for partial nuclear localization of PIPkin-I and PIPkin-II, and some PtdIns(4,5)P₂ is made and undoubtedly functions in the nucleus – but how it is made and what it does remain unclear.

The individual PI3K subfamilies phosphorylate different phosphoinositides. The functions of Type II PI3Ks remain uncertain. Type III PI3Ks (PI3K-IIIs) convert PtdIns to PtdIns3P. This lipid is essential for vesicle trafficking from the trans-Golgi to vacuole/lysosome compartments. It exerts most or all of its effects through interactions with secretory/endosomal target proteins that include either PtdIns3P-specific FYVE domains and/or phox (or PX) domains (see Table). This PtdIns3P is also the substrate for the synthesis by PIPkin-IIIs of PtdIns(3,5)P₂. Type I PI3Ks (PI3K-Is) phosphorylate PtdIns(4,5)P₂ to PtdIns(3,4,5)P₃ and thereby transmit signals from extracellular stimuli. PI3K-IAs are mainly activated by receptor tyrosine kinases (e.g. the insulin receptor and many growth factor receptors), and PI3K-IBs by some G protein-coupled receptors (particularly, in hemopoietic cells, by receptors that influence motility and the bactericidal oxidative burst of neutrophils).

PtdIns(3,4,5)P₃ is a remarkable membrane-associated second messenger molecule. It appears to have many direct target proteins, each of which it interacts with through highly PtdIns(3,4,5)P₃-selective pleckstrin homology (PH) domains. Some of these are protein kinases (e.g. PDK1, PKB, Btk, Itk), and others include regulators of small GTPases (GTP/GDP exchange factors and GTPase-activating factors). The PtdIns(3,4,5)P₃-activated protein kinases have numerous substrates, and they often initiate downstream protein kinase cascades.

In addition, there is a diverse group of ‘Type IV PI3K-related kinases’ (or PIKKs) – comprising TOR (target of rapamycin) proteins, DNA-dependent protein kinase (DNA-PK) and ATM (Ataxia Telangiectasia Mutated). Although their kinase domains are related to those of bona fide PI3Ks, they are probably protein kinases.

The cellular regulation of cellular phosphoinositide levels, and of the sizes of the phosphoinositide pools at particular intracellular sites, are as dependent on the activities of the diverse PPIn phosphatases as on the kinases discussed above. Two of these are PTEN, the loss of which makes cells prone to become malignant, and the myotubularins (MTM1-), loss of some of which lead to other conditions.

The Table below contains accepted modulators and additional information. For a list of additional products, see the "Similar Products" section below.

Abbreviations

PI3K: Phosphoinositide 3-kinase
PIK: PtdIns kinase
PIP: Phosphatidylinositol phosphate
PIPkin: PtdInsP kinase
PPI: Polyphosphoinositides
PtdIns: Phosphatidylinositol
PtdIns3P: PtdIns 3-phosphate
PtdIns4P: PtdIns 4-phosphate
PtdIns5P: PtdIns 5-phosphate
PtdIns(3,4)P₂: PtdIns 3,4-bisphosphate
PtdIns(3,5)P₂: PtdIns 3,5-bisphosphate
PtdIns(4,5)P₂: PtdIns 4,5-bisphosphate
PtdIns(3,4,5)P₃: PtdIns 3,4,5-trisphosphate
Ins(1,4,5)P₃: Inositol 1,4,5-trisphosphate