PI Metabolism (Homo sapiens)
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Phosphatidylinositol (PI), a membrane phospholipid, can be reversibly phosphorylated at the 3, 4, and 5 positions of the inositol ring to generate seven phosphoinositides: phosphatidylinositol 3-phosphate (PI3P), phosphatidylinositol 4-phosphate (PI4P), phosphatidylinositol 5-phosphate (PI5P), phosphatidylinositol 3,4-bisphosphate PI(3,4)P2, phosphatidylinositol 4,5-bisphosphate PI(4,5)P2, phosphatidylinositol 3,5-bisphosphate PI(3,5)P2, and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3). These seven phosphoinositides, which are heterogeneously distributed within cells, can serve as signature components of different intracellular compartment membranes and so help to mediate specificity of membrane interactions. Phosphoinositide levels are tightly regulated spatially and temporally by the action of various kinases and phosphatases whilst PI(4,5)P2 is also a substrate for phospholipase C. The differential localisation of each of these enzymes on specific compartment membranes ensures maintenance of the heterogeneous distribution of phosphoinositides despite the continuous membrane flow from one compartment to another.
PI is primarily synthesised in the endoplasmic reticulum from where the phospholipid is exported to other compartments via membrane traffic or via cytosolic phospholipid transfer proteins. Phosphorylation of PI to PI4P primarily occurs in the Golgi, where PI4P plays an important role in the biogenesis of transport vesicles such as the secretory vesicle involved in its transport to the plasma membrane. At this place, PI4P has a major function acting as a precursor of PI(4,5)P2, which is located predominantly at this membrane. PI(4,5)P2 binds and regulates a wide range of proteins that function on the cell surface and serves as a precursor for second messengers. Additionally, it helps define this membrane as a target for secretory vesicles, functions as a coreceptor in endocytic processes, and functions as a cofactor for actin nucleation.
At the plasma membrane, PI(4,5)P2 is further phosphorylated to PI(3,4,5)P3, another phosphoinositide with important signalling functions including stimulating cell survival and proliferation. The inositol 3-phosphatase, phosphatase and tensin homolog (PTEN) regenerates PI(4,5)P2, while the 5-phosphatases convert PI(3,4,5)P3 into the phosphoinositide, PI(3,4)P2, propagating the signal initiated by PI(3,4,5)P3. PI(3,4)P2 is further dephosphorylated in the endocytic pathway by inositol 4-phosphatases to PI3P, the signature phosphoinositide of the early endosomal compartment and a ligand for numerous endosomal proteins. However, the bulk of PI3P is generated directly in the endosomes by phosphorylation of PI. The subsequent endosomal phosphorylation of PI3P to PI(3,5)P2 is believed to generate docking sites for recruitment of cytosolic factors responsible for the control of outgoing traffic from the endosomes. The main localisation and function of the low abundance phosphoinositide PI5P, that can be generated by several pathways, remains to be determined (Krauss & Haucke 2007, Leventis & Grinstein 2010, Roth 2004, Gees et al. 2010, De Matteis & Godi 2004, van Meer et al. 2008, Vicinanza et al. 2008, Lemmon 2008, Kutaleladze 2010, Robinson & Dixon 2006, Blero et al. 2007, Liu & Bankaitis 2010, McCrea & De Camilli 2009, Vicinanza et al. 2008, Di Paolo & De Camilli, 2006). View original pathway at Reactome.
PI is primarily synthesised in the endoplasmic reticulum from where the phospholipid is exported to other compartments via membrane traffic or via cytosolic phospholipid transfer proteins. Phosphorylation of PI to PI4P primarily occurs in the Golgi, where PI4P plays an important role in the biogenesis of transport vesicles such as the secretory vesicle involved in its transport to the plasma membrane. At this place, PI4P has a major function acting as a precursor of PI(4,5)P2, which is located predominantly at this membrane. PI(4,5)P2 binds and regulates a wide range of proteins that function on the cell surface and serves as a precursor for second messengers. Additionally, it helps define this membrane as a target for secretory vesicles, functions as a coreceptor in endocytic processes, and functions as a cofactor for actin nucleation.
At the plasma membrane, PI(4,5)P2 is further phosphorylated to PI(3,4,5)P3, another phosphoinositide with important signalling functions including stimulating cell survival and proliferation. The inositol 3-phosphatase, phosphatase and tensin homolog (PTEN) regenerates PI(4,5)P2, while the 5-phosphatases convert PI(3,4,5)P3 into the phosphoinositide, PI(3,4)P2, propagating the signal initiated by PI(3,4,5)P3. PI(3,4)P2 is further dephosphorylated in the endocytic pathway by inositol 4-phosphatases to PI3P, the signature phosphoinositide of the early endosomal compartment and a ligand for numerous endosomal proteins. However, the bulk of PI3P is generated directly in the endosomes by phosphorylation of PI. The subsequent endosomal phosphorylation of PI3P to PI(3,5)P2 is believed to generate docking sites for recruitment of cytosolic factors responsible for the control of outgoing traffic from the endosomes. The main localisation and function of the low abundance phosphoinositide PI5P, that can be generated by several pathways, remains to be determined (Krauss & Haucke 2007, Leventis & Grinstein 2010, Roth 2004, Gees et al. 2010, De Matteis & Godi 2004, van Meer et al. 2008, Vicinanza et al. 2008, Lemmon 2008, Kutaleladze 2010, Robinson & Dixon 2006, Blero et al. 2007, Liu & Bankaitis 2010, McCrea & De Camilli 2009, Vicinanza et al. 2008, Di Paolo & De Camilli, 2006). View original pathway at Reactome.
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EPH receptors are the largest known family of receptor tyrosine kinases (RTKs), with fourteen total receptors divided into either A- or B-subclasses: EPHA (1-8 and 10) and EPHB (1-4 and 6). EPH receptors can have overlapping functions, and loss of one receptor can be partially compensated for by another EPH receptor that has similar expression pattern and ligand-binding specificities. EPH receptors have an N-terminal extracellular domain through which they bind to ephrin ligands, a short transmembrane domain, and an intracellular cytoplasmic signaling structure containing a canonical tyrosine kinase catalytic domain as well as other protein interaction sites. Ephrins are also sub-divided into an A-subclass (A1-A5), which are tethered to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor, and a B-subclass (B1-B3), members of which have a transmembrane domain and a short, highly conserved cytoplasmic tail lacking endogenous catalytic activity. The interaction between EPH receptors and its ligands requires cell-cell interaction since both molecules are membrane-bound. Close contact between EPH receptors and EFNs is required for signaling to occur. EPH/EFN-initiated signaling occurs bi-directionally into either EPH- or EFN-expressing cells or axons. Signaling into the EPH receptor-expressing cell is referred as the forward signal and signaling into the EFN-expressing cell, the reverse signal. (Dalva et al. 2000, Grunwald et al. 2004, Davy & Robbins 2000, Cowan et al. 2004)
PI(3,4)P2,
PI(3,4,5)P3Activity through
AcetylationAnnotated Interactions
PI(3,4)P2,
PI(3,4,5)P3The following lists the above proteins with their corresponding literature references: PIP5K1A (Zhang et al. 1997, Tolias et al. 1998), PIP5K1B (Zhang et al. 1997, Tolias et al. 1998), and PIP5K1C (Wenk et al. 2001, Di Paolo et al. 2002, Krauss et al. 2003).
The following lists the above proteins with their corresponding literature references: PIP4K2A (Rameh et al. 1997, Clarke et al. 2008, Clarke and Irvine 2013), PIP4K2B (Rameh et al. 1997, Clarke and Irvine 2013) and PIP4K2C (Clarke and Irvine 2013, Clarke et al. 2015).
The following lists the above proteins with their corresponding literature references: MTM1 (Cao et al. 2007, Cao et al. 2008, Tsujita et al. 2004, Tronchere et al. 2004, Kim et al. 2002); MTMR2 (Cao et al. 2008, Kim et al. 2002); MTMR4 (Lorenzo et al. 2006); and MTMR7 (Mochizuki & Majerus 2003, Lorenzo et al. 2006).
The following lists the above proteins with their corresponding literature references: PIK3C3:PIK3R4 complex (Panaretou et al. 1997, Volinia et al. 1995, Cao et al. 2007) and PIK3C2A (Arcaro et al. 2000, Domin et al. 2000).
The following lists the above proteins with their corresponding literature references: PIK3C3:PIK2R4 (Panaretou et al. 1997, Volinia et al. 1995) and PIK3C2A (Arcaro et al. 2000, Domin et al. 2000).
The following lists the above proteins with their corresponding literature references: PIK3C3:PIK3R4 (Panaretou et al. 1997, Volinia et al. 1995, Cao et al. 2007) and PIK3C2A (Arcaro et al. 2000, Domin et al. 2000).
The following lists the above proteins with their corresponding literature references: PIK3CA:PIK3R1, PIK3CA:PIK3R2, PIK3CA:PIK3R3 (Dey et al. 1998, Vanhaesebroeck et al. 1997, Meier et al. 2004); PIK3CB:PIK3R1, PIK3CB:PIK3R2, PIK3CB:PIK3R3 (Meier et al. 2004); PIK3CD:PIK3R1, PIK3CD:PIK3R2, PIK3CD:PIK3R3 (Vanhaesebroeck et al. 1997, Meier et al. 2004); and PIK3CG:PIK3R5, PIK3CG:PIK3R6 (Voigt et al. 2006, Suire et al. 2005, Stoyanov et al. 1995).
The following lists the above proteins with their corresponding literature references: MTM1 (Cao et al. 2007, Cao et al. 2008, Tsujita et al. 2004, Tronchere et al. 2004), MTMR2 (Cao et al. 2008), MTMR4 (Lorenzo et al. 2006), and MTMR7 (Mochizuki & Majerus 2003, Lorenzo et al. 2006).
The following lists the above proteins with their corresponding literature references: PIP5K1A (Halstead et al. 2006, Zhang et al. 1997), PIP5K1B (Zhang et al. 1997), and PIP5K1C (Di Paolo et al. 2002).
This reaction is of particular interest because its regulation by small GTPases of the RHO and ARF families, not yet annotated here, ties the process of phosphatidylinositol phosphate biosynthesis to regulation of the actin cytoskeleton and vesicular trafficking, and hence to diverse aspects of cell motility and signalling (Oude Weernink et al. 2004, 2007).
The following lists the above proteins with their corresponding literature references: MTM1 (Cao et al. 2007, Cao et al. 2008), MTMR2 (Cao et al. 2008), and MTMR4 (Lorenzo et al. 2006).
The following lists the above proteins with their corresponding literature references: PIK3C2A (Arcaro et al. 2000); PIK3C2B (Arcaro et al. 2000, Arcaro et al. 1998); PIK3C2G (Misawa et al. 1998, Ono et al. 1998); PIK3CA:PIK3R1, PIK3CA:PIK3R2, PIK3CA:PIK3R3 (Vanhaesebroeck et al. 1997); PIK3CB:PIK3R1, PIK3CB:PIK3R2, PIK3CB:PIK3R3 (Meier et al. 2004, Guo et al. 1997); PIK3CD:PIK3R1, PIK3CD:PIK3R2, PIK3CD:PIK3R3 (Vanhaesebroeck et al. 1997); and PIK3CG:PIK3R5, PIK3CG:PIK3R6 (Suire et al. 2005, Stoyanov et al. 1995).
The following lists the above proteins with their corresponding literature references: MTM1 (Cao et al. 2007, Cao et al. 2008, Kim et al. 2002), MTMR2 (Cao et al. 2008, Kim et al. 2002), and MTMR4 (Lorenzo et al. 2006, Zhao et al. 2001).
The following lists the above proteins with their corresponding literature references: PIP4K2A (Zhang et al. 1997, Rameh et al. 1997, Clarke et al. 2008), PIP4K2B (Zhang et al. 1997, Rameh et al. 1997), PIP5K1A (Zhang et al. 1997, Tolias et al. 1998), PIP5K1B (Zhang et al. 1997, Tolias et al. 1998), and PIP5K1C (Wenk et al. 2001, Di Paolo et al. 2002).
Early studies indicated that magnesium ion, Mg2+, was needed for the catalytic activity of PTEN isolated from bovine thymus (Kabuyama et al. 1996). Subsequent studies have shown that PTEN was catalytically active in buffers free of magnesium and magnesium was not detected as part of the PTEN crystal (Lee et al. 1999).
The following lists the above proteins with their corresponding literature references: SYNJ1 (Johenning et al. 2004, Haffner et al. 1997, Guo et al. 1999, Mani et al. 2007), SYNJ2 (Malecz et al. 2000), INPP5K (Injuin et al. 2000, Gurung et al. 2003), and INPP5J (Gurung et al. 2003, Mochizuki & Takenawa 1999).
Early studies indicated that magnesium ion, Mg2+, was needed for the catalytic activity of PTEN isolated from bovine thymus (Kabuyama et al. 1996). Subsequent studies have shown that PTEN was catalytically active in buffers free of magnesium and magnesium was not detected as part of the PTEN crystal (Lee et al. 1999).
(PIP3, PtdIns(3,4,5)P) is generated by the action of phosphoinositide 3-kinase (PI3K) in response to growth factors and insulin and regulates a range of cellular processes. Proteins containing the plekstrin homology (PH) domain can interact specifically with PIP3 or its immediate breakdown product, phosphatidylinositol 3,4-diphosphate (PIP2, PtdIns(3,4)P). Proteins with a PH domain have also been found to bind to PIs other than PIP3 or PIP2. Pleckstrin homology domain-containing family A member 4 (PLEKHA4 aka PEPP1) is able to specifically bind phosphatidylinositol 3-phosphate (PI3P) but not other phosphoinositides (Dowler et al. 2000). Two related isoforms of
PLEKHA4, PLEKHA5 and 6 (PEPP2 and PEPP3), possess a very similar PH domain sequence, indicating that they may also interact with PI3P (Dowler et al. 2000, Yamada et al. 2012). These proteins may function as adaptor molecules since they possess no obvious catalytic moieties.