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Gene Description
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Phosphoinositide-specific phospholipase C plays a significant role in transmembrane signaling. In response to extracellular stimuli, such as hormones, growth factors, and neurotransmitters, PLC hydrolyzes phosphatidylinositol 4, 5-bisphosphate to generate two secondary messengers: inositol 1, 4, 5-triphosphate and diacylglycerol. At least four families of PLCs have been identified: PLCβ, PLCγ, PLCδ, and PLCε. Phosphorylation is one of the key mechanisms that regulate the activity of PLC. PLCγ is activated by both receptor and non-receptor tyrosine kinases. PLCγ forms a complex with EGF and PDGF receptors, which leads to the phosphorylation of PLCγ at Tyr771, 783, and 1248. Phosphorylation by Syk at Tyr783 activates the enzymatic activity of PLCγ1. PLCγ2 is engaged in antigen-dependent signaling in B cells and collagen-dependent signaling in platelets. Phosphorylation by Btk or Lck at Tyr753, 759, 1197, and 1217 is correlated with PLCγ2 activity.Two mammalian PLCγ isoforms have been cloned and characterized. Like other PLC-family members, PLCγ1 and PLCγ2 contain calcium-binding and lipid-interacting domains necessary for their enzymatic activity and substrate recognition., iquely, PLCγ isoforms have additional, conserved SH2 and SH3 domains critical for their functions as signaling molecules and scaffolding proteins. Upon growth factor stimulation, PLCγ1 is recruited to phosphotyrosine residues within the cytoplasmic tail of many RTKs where it serves as a substrate for the RTK and provides docking sites for additional proteins involved in RTK signaling. PLCγ1 and γ2 can also be activated downstream of receptors lacking intrinsic tyrosine kinase activity. This has been reported downstream of multiple G protein-coupled receptors and the T cell receptor in which tyrosine kinases of the Src, Syk, and Tec families serve to bind, phosphorylate, and activate PLCγ. Phosphorylation at tyrosine residues by both receptor and non-receptor tyrosine kinases results in robust activation of PLCγ1 activity, leading to generation of second messengers. In response to agonists, PLCγ1 is phosphorylated on Tyr783, Tyr711, and Tyr1253 resulting in robust PI-4, 5-P2 hydrolysis. Interestingly recent evidence suggests a role for tyrosine kinase-independent regulation of PLCγ in some systems. For example, in response to EGF, proline-rich regions of Akt interact with the SH3 domain of PLCγ1 resulting in association of the two enzymes, phosphorylation of PLCγ1 at Ser1248, and enhanced cellular motility. This finding demonstrates that PLCγ1 can function as a "scaffold" between RTKs and Akt, thereby establishing a mechanism by which the Akt signaling pathway cross-talks with tyrosine kinases. However, the mechanism and functional significance of phosphorylation at Ser1248 remains to be fully clarified, as it has also been shown that PKA-mediated phosphorylation at this site is inhibitory to PLCγ1 tyrosine phosphorylation and phospholipase activity in CD3-treated Jurkat cells, suggesting that Ser1248 may be an allosteric regulator of PLCγ1 activity. |
