Summary: Phosphatidylinositol 3- and 4-kinase
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Phosphoinositide 3-kinase Edit Wikipedia article
|Phosphatidylinositol 3- and 4-kinase|
PI3 Kinase 110 gamma bound to the inhibitor PIK-93 (yellow).
Phosphatidylinositide 3-kinases (PI 3-kinases, PI3Ks, PI(3)Ks, or PI-3Ks) are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which in turn are involved in cancer. PI3Ks are a family of related intracellular signal transducer enzymes capable of phosphorylating the 3 position hydroxyl group of the inositol ring of phosphatidylinositol (PtdIns). They are also known as phosphatidylinositol-3-kinases or phosphoinositide 3-kinases. The pathway, with oncogene PIK3CA and tumor suppressor PTEN (gene), is implicated in insensitivity of cancer tumors to insulin and IGF1, in calorie restriction.
The discovery of PI 3-kinases by Lewis Cantley and colleagues began with their identification of a previously unknown phosphoinositide kinase associated with the polyoma middle T protein. They observed unique substrate specificity and chromatographic properties of the products of the lipid kinase, leading to the discovery that this phosphoinositide kinase had the unprecedented ability to phosphorylate phosphoinositides on the 3' position of the inositol ring. Subsequently, Cantley and colleagues demonstrated that in vivo the enzyme prefers PtdIns(4,5)P2 as a substrate, producing the novel phosphoinositide PtdIns(3,4,5)P3.
PI3Ks interact with the IRS (Insulin receptor substrate) in order to regulate glucose uptake through a series of phosphorylation events.
The phosphoinositol-3-kinase family is divided into three different classes: Class I, Class II, and Class III. The classifications are based on primary structure, regulation, and in vitro lipid substrate specificity.
Class I PI3Ks are responsible for the production of Phosphatidylinositol 3-phosphate (PI(3)P), Phosphatidylinositol (3,4)-bisphosphate (PI(3,4)P2), and Phosphatidylinositol (3,4,5)-trisphosphate (PI(3,4,5)P3,  The PI3K is activated by G protein-coupled receptors and tyrosine kinase receptors.
Class I PI3K are heterodimeric molecules composed of a regulatory and a catalytic subunit; they are further divided between IA and IB subsets on sequence similarity. Class IA PI3K is composed of a heterodimer between a p110 catalytic subunit and a p85 regulatory subunit. There are five variants of the p85 regulatory subunit, designated p85α, p55α, p50α, p85β, and p85γ. There are also three variants of the p110 catalytic subunit designated p110α, β, or δ catalytic subunit. The first three regulatory subunits are all splice variants of the same gene (Pik3r1), the other two being expressed by other genes (Pik3r2 and Pik3r3, p85β, and p55γ, respectively). The most highly expressed regulatory subunit is p85α; all three catalytic subunits are expressed by separate genes (Pik3ca, Pik3cb, and Pik3cd for p110α, p110β, and p110δ, respectively). The first two p110 isoforms (α and β) are expressed in all cells, but p110δ is expressed primarily in leukocytes, and it has been suggested that it evolved in parallel with the adaptive immune system. The regulatory p101 and catalytic p110γ subunits comprise the type IB PI3K and are encoded by a single gene each.
The p85 subunits contain SH2 and SH3 domains (Online 'Mendelian Inheritance in Man' (OMIM) 171833). The SH2 domains bind preferentially to phosphorylated tyrosine residues in the amino acid sequence context Y-X-X-M.
Classes II and III
Class II comprises three catalytic isoforms (C2α, C2β, and C2γ), but, unlike Classes I and III, no regulatory proteins. Class II catalyse the production of PI(3)P from PI and PI(3,4)P2 from PIP; however, little is known about their role in immune cells. C2α and C2β are expressed through the body, however expression of C2γ is limited to hepatocytes. The distinct feature of Class II PI3Ks is the C-terminal C2 domain. This domain lacks critical Asp residues to coordinate binding of Ca2+, which suggests class II PI3Ks bind lipids in a Ca2+-independent manner.
Class III produces only PI(3)P from PI  but are more similar to Class I in structure, as they exist as a heterodimers of a catalytic (Vps34) and a regulatory (Vps15/p150) subunits. Class III seems to be primarily involved in the trafficking of proteins and vesicles. There is, however, evidence to show that they are able to contribute to the effectiveness of several process important to immune cells, not least phagocytosis.
|class 2||PIK3C2A||PI3K, class 2, alpha polypeptide||PI3K-C2α||184.108.40.206|
|PIK3C2B||PI3K, class 2, beta polypeptide||PI3K-C2β|
|PIK3C2G||PI3K, class 2, gamma polypeptide||PI3K-C2γ|
|class 3||PIK3C3||PI3K, class 3||Vps34||220.127.116.11|
|class 1 catalytic||PIK3CA||PI3K, catalytic, alpha polypeptide||p110-α||18.104.22.168|
|PIK3CB||PI3K, catalytic, beta polypeptide||p110-β|
|PIK3CG||PI3K, catalytic, gamma polypeptide||p110-γ|
|PIK3CD||PI3K, catalytic, delta polypeptide||p110-δ|
|class 1 regulatory||PIK3R1||PI3K, regulatory subunit 1 (alpha)||p85-α||N/A|
|PIK3R2||PI3K, regulatory subunit 2 (beta)||p85-β|
|PIK3R3||PI3K, regulatory subunit 3 (gamma)||p55-γ|
|PIK3R4||PI3K, regulatory subunit 4||p150|
|PIK3R5||PI3K, regulatory subunit 5||p101|
|PIK3R6||PI3K, regulatory subunit 6||p87|
The various 3-phosphorylated phosphoinositides that are produced by PI 3-kinases (PtdIns3P, PtdIns(3,4)P2, PtdIns(3,5)P2, and PtdIns(3,4,5)P3) function in a mechanism by which an assorted group of signalling proteins, containing PX domain, pleckstrin homology domains (PH domains), FYVE domains and other phosphoinositide-binding domains, are recruited to various cellular membranes.
PI 3-kinases have been linked to an extraordinarily diverse group of cellular functions, including cell growth, proliferation, differentiation, motility, survival and intracellular trafficking. Many of these functions relate to the ability of class I PI 3-kinases to activate protein kinase B (PKB, aka Akt) as in the PI3K/AKT/mTOR pathway. The p110δ and p110γ isoforms regulate different aspects of immune responses. PI 3-kinases are also a key component of the insulin signaling pathway. Hence there is great interest in the role of PI 3-kinase signaling in Diabetes mellitus.
The pleckstrin homology domain of AKT binds directly to PtdIns(3,4,5)P3 and PtdIns(3,4)P2, which are produced by activated PI 3-kinase. Since PtdIns(3,4,5)P3 and PtdIns(3,4)P2 are restricted to the plasma membrane, this results in translocation of AKT to the plasma membrane. Likewise, the phosphoinositide-dependent kinase-1 (PDK1 or, rarely referred to as PDPK1) also contains a pleckstrin homology domain that binds directly to PtdIns(3,4,5)P3 and PtdIns(3,4)P2, causing it to also translocate to the plasma membrane upon activation of PI 3-kinase. The colocalization of activated PDK1 and AKT allows AKT to become phosphorylated by PDK1 on threonine 308, leading to partial activation of AKT. Full activation of AKT occurs upon phosphorylation of serine 473 by the TORC2 complex of the mTOR protein kinase. (The nomenclature can be confusing. Note that PDK1 also refers to the unrelated enzyme Pyruvate dehydrogenase kinase, isozyme 1. Similarly, TORC2 also refers to the unrelated transcription factor Transducer of Regulated CREB activity 2, which has recently been renamed CREB-regulated transcription coactivator 2 (CRTC2) to reduce the confusion). The "PI3-k/AKT" signaling pathway has been shown to be required for an extremely diverse array of cellular activities - most notably cellular proliferation and survival. The phosphatidylinositol 3-kinase/protein kinase B pathway is stimulated in protection of astrocytes from ceramide-induced apoptosis.
Many other proteins have been identified that are regulated by PtdIns(3,4,5)P3, including Bruton's tyrosine kinase (BTK), General Receptor for Phosphoinositides-1 (GRP1), and the O-linked N-acetylglucosamine (O-GlcNAc) transferase.
The class IA PI 3-kinase p110α is mutated in many cancers. Many of these mutations cause the kinase to be more active. The PtdIns(3,4,5)P3 phosphatase PTEN that antagonises PI 3-kinase signaling is absent from many tumours. Hence, PI 3-kinase activity contributes significantly to cellular transformation and the development of cancer.
Learning and memory
PI3K has also been implicated in Long-term potentiation (LTP). Whether it is required for the expression or the induction of LTP is still debated. In mouse hippocampal CA1 neurons, PI3K is complexed with AMPA Receptors and compartmentalized at the postsynaptic density of glutamatergic synapses. PI3K is phosphorylated upon NMDA Receptor-dependent CaMKII activity, and it then facilitates the insertion of AMPA-R GluR1 subunits into the plasma membrane. This suggests that PI3K is required for the expression of LTP. Furthermore, PI3K inhibitors abolished the expression of LTP in rat hippocampal CA1, but do not affect its induction. Notably, the dependence of late-phase LTP expression on PI3K seems to decrease over time.
However, another study found that PI3K inhibitors suppressed the induction, but not the expression, of LTP in mouse hippocampal CA1. The PI3K pathway also recruits many other proteins downstream, including mTOR, GSK3β, and PSD-95. The PI3K-mTOR pathway leads to the phosphorylation of p70S6K, a kinase that facilitates translational activity  , further suggesting that PI3K is required for the protein-synthesis phase of LTP induction instead.
PI 3-kinases as protein kinases
Many of the PI 3-kinases appear to have a serine/threonine kinase activity in vitro; however, it is unclear whether this has any role in vivo.
In addition to the class I – class III PI 3-kinases there is a group of more distantly related enzymes that are sometimes referred to as class IV PI 3-kinases. The class IV PI 3-kinases family is composed of ataxia telangiectasia mutated (ATM), ataxia telangiectasia and Rad3 related (ATR), DNA-dependent protein kinase (DNA-PK) and mammalian Target Of Rapamycin (mTOR). These members of the PI 3-kinase superfamily are protein serine/threonine kinases.
PI 3-kinases inhibitors as therapeutics
As wortmannin and LY294002 are broad inhibitors against PI 3-kinases and a number of unrelated proteins at higher concentrations they are too toxic to be used as therapeutics. A number of pharmaceutical companies have recently been working on PI 3-kinase isoform specific inhibitors including the class I PI 3-kinase, p110δ isoform specific inhibitors, IC486068 and IC87114, ICOS Corporation..GDC-0941 is a highly selective inhibitor of p110α with little activity against mTOR.
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- Sanna PP, Cammalleri M, Berton F, et al. (May 2002). "Phosphatidylinositol 3-kinase is required for the expression but not for the induction or the maintenance of long-term potentiation in the hippocampal CA1 region". Journal of Neuroscience 22 (9): 3359–65. PMID 11978812.
- Karpova A, Sanna PP, Behnisch T (February 2006). "Involvement of multiple phosphatidylinositol 3-kinase-dependent pathways in the persistence of late-phase long term potentiation expression". Neuroscience 137 (3): 833–41. doi:10.1016/j.neuroscience.2005.10.012. PMID 16326012.
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- Toker A, Cantley LC (June 1997). "Signalling through the lipid products of phosphoinositide-3-OH kinase". Nature 387 (6634): 673–6. doi:10.1038/42648. PMID 9192891.
- Cammalleri M, Lütjens R, Berton F, et al. (November 2003). "Time-restricted role for dendritic activation of the mTOR-p70S6K pathway in the induction of late-phase long-term potentiation in the CA1". Proceedings of the National Academy of Sciences of the United States of America 100 (24): 14368–73. doi:10.1073/pnas.2336098100. PMC 283598. PMID 14623952.
- Vanhaesebroeck B, Leevers S, Ahmadi K, Timms J, Katso R, Driscoll P, Woscholski R, Parker P, Waterfield M (2001). "Synthesis and function of 3-phosphorylated inositol lipids". Annu Rev Biochem 70: 535–602. doi:10.1146/annurev.biochem.70.1.535. PMID 11395417. 
- Schild C, Wirth M, Reichert M, Schmid RM, Saur D, Schneider G (July 2009). "PI3K signaling maintains c-myc expression to regulate transcription of E2F1 in pancreatic cancer cells". Mol. Carcinog. 48 (12): 1149–58. doi:10.1002/mc.20569. PMID 19603422.
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- Quaresma AJ, Sievert R, Nickerson JA (2013). "Regulation of mRNA export by the PI3 kinase/AKT signal transduction pathway". Mol. Biol. Cell. April (8): 1208–21. doi:10.1091/mbc.E12-06-0450. PMC 3623641. PMID 23427269.
- Eukaryotic Linear Motif resource motif class MOD_PIKK_1
- Proteopedia Phosphoinositide_3-Kinases to explore the structure in interactive 3D
- PI-3 Kinase at the US National Library of Medicine Medical Subject Headings (MeSH)
- PI3K/Akt Signaling Pathway
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This tab holds annotation information from the InterPro database.
InterPro entry IPR000403
Phosphatidylinositol 3-kinase (PI3-kinase) (EC) [PUBMED:1322797] is an enzyme that phosphorylates phosphoinositides on the 3-hydroxyl group of the inositol ring. The three products of PI3-kinase - PI-3-P, PI-3,4-P(2) and PI-3,4,5-P(3) function as secondary messengers in cell signalling. Phosphatidylinositol 4-kinase (PI4-kinase) (EC) [PUBMED:8194527] is an enzyme that acts on phosphatidylinositol (PI) in the first committed step in the production of the secondary messenger inositol-1'4'5'-trisphosphate. This domain is also present in a wide range of protein kinases, involved in diverse cellular functions, such as control of cell growth, regulation of cell cycle progression, a DNA damage checkpoint, recombination, and maintenance of telomere length. Despite significant homology to lipid kinases, no lipid kinase activity has been demonstrated for any of the PIK-related kinases [PUBMED:12456783].
The PI3- and PI4-kinases share a well conserved domain at their C-terminal section; this domain seems to be distantly related to the catalytic domain of protein kinases [PUBMED:8387896, PUBMED:12151228]. The catalytic domain of PI3K has the typical bilobal structure that is seen in other ATP-dependent kinases, with a small N-terminal lobe and a large C-terminal lobe. The core of this domain is the most conserved region of the PI3Ks. The ATP cofactor binds in the crevice formed by the N-and C-terminal lobes, a loop between two strands provides a hydrophobic pocket for binding of the adenine moiety, and a lysine residue interacts with the alpha-phosphate. In contrast to protein kinases, the PI3K loop which interacts with the phosphates of the ATP and is known as the glycine-rich or P-loop, contains no glycine residues. Instead, contact with the ATP -phosphate is maintained through the side chain of a conserved serine residue.
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Molecular function||phosphotransferase activity, alcohol group as acceptor (GO:0016773)|
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1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
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|Seed source:||Prosite & Pfam-B_6771 (Rlease 7.6)|
|Author:||Sonnhammer ELL, Finn RD|
|Number in seed:||43|
|Number in full:||4451|
|Average length of the domain:||225.50 aa|
|Average identity of full alignment:||21 %|
|Average coverage of the sequence by the domain:||15.05 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 23193494 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||22|
|Download:||download the raw HMM for this family|
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Some species in the taxonomic tree may not have one or more of the main eight levels that we display. For example, Bos taurus is not assigned an order in the NCBI taxonomic tree. In such cases we mark the omitted level with, for example, "No order", in both the tooltip and the lineage summary.
Unmapped species names
The tree is built by looking at each sequence in the full alignment for the family. We take the name of the species given by UniProt and try to map that to the full taxonomic tree from NCBI. In some cases, the name chosen by UniProt does not map to any node in the NCBI tree, perhaps because the chosen name is listed as a synonym or a misspelling in the NCBI taxonomy.
So that these nodes are not simply omitted from the sunburst tree, we group them together in a separate branch (or segment of the sunburst tree). Since we cannot determine the lineage for these unmapped species, we show all levels between the superkingdom and the species as "uncategorised".
Since we reduce the species tree to only the eight main taxonomic levels, sequences that are mapped to the sub-species level in the tree would not normally be shown. Rather than leave out these species, we map them instead to their parent species. So, for example, for sequences belonging to one of the Vibrio cholerae sub-species in the NCBI taxonomy, we show them instead as belonging to the species Vibrio cholerae.
Too many species/sequences
For large species trees, you may see blank regions in the outer layers of the sunburst. These occur when there are large numbers of arcs to be drawn in a small space. If an arc is less than approximately one pixel wide, it will not be drawn and the space will be left blank. You may still be able to get some information about the species in that region by moving your mouse across the area, but since each arc will be very small, it will be difficult to accurately locate a particular species.
The tree shows the occurrence of this domain across different species. More...
We show the species tree in one of two ways. For smaller trees we try to show an interactive representation, which allows you to select specific nodes in the tree and view them as an alignment or as a set of Pfam domain graphics.
Unfortunately we have found that there are problems viewing the interactive tree when the it becomes larger than a certain limit. Furthermore, we have found that Internet Explorer can become unresponsive when viewing some trees, regardless of their size. We therefore show a text representation of the species tree when the size is above a certain limit or if you are using Internet Explorer to view the site.
If you are using IE you can still load the interactive tree by clicking the "Generate interactive tree" button, but please be aware of the potential problems that the interactive species tree can cause.
For all of the domain matches in a full alignment, we count the number that are found on all sequences in the alignment. This total is shown in the purple box.
We also count the number of unique sequences on which each domain is found, which is shown in green. Note that a domain may appear multiple times on the same sequence, leading to the difference between these two numbers.
Finally, we group sequences from the same organism according to the NCBI code that is assigned by UniProt, allowing us to count the number of distinct sequences on which the domain is found. This value is shown in the pink boxes.
We use the NCBI species tree to group organisms according to their taxonomy and this forms the structure of the displayed tree. Note that in some cases the trees are too large (have too many nodes) to allow us to build an interactive tree, but in most cases you can still view the tree in a plain text, non-interactive representation. Those species which are represented in the seed alignment for this domain are highlighted.
You can use the tree controls to manipulate how the interactive tree is displayed:
- show/hide the summary boxes
- highlight species that are represented in the seed alignment
- expand/collapse the tree or expand it to a given depth
- select a sub-tree or a set of species within the tree and view them graphically or as an alignment
- save a plain text representation of the tree
Please note: for large trees this can take some time. While the tree is loading, you can safely switch away from this tab but if you browse away from the family page entirely, the tree will not be loaded.
There are 4 interactions for this family. More...
We determine these interactions using iPfam, which considers the interactions between residues in three-dimensional protein structures and maps those interactions back to Pfam families. You can find more information about the iPfam algorithm in the journal article that accompanies the website.
For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the PI3_PI4_kinase domain has been found. There are 95 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.
Loading structure mapping...