View source for Phosphoinositide phospholipase C

{{More citations needed|date=July 2011}}
{{Pfam_box
| Symbol = PI-PLC-X
| Name = Phosphatidylinositol-specific phospholipase C
| image = Phospholipase C-1.png
| width = 220px
| caption =Phospholipase Cδ-1
| Pfam= PF00388
| Pfam_clan = CL0384
| InterPro= IPR000909
| SMART=  PLCXc
| PROSITE = PDOC50007
| SCOP = 1gym
| TCDB =
| OPM family= 118
| OPM protein= 1djx
| CDD = cd00137
| PDB=
}}

{{infobox enzyme
| Name       = phosphoinositide phospholipase C
| EC_number  = 3.1.4.11
| CAS_number = 37213-51-7
| GO_code    = 0004435
| image      = 2zkm.jpg
| width      = 270
| caption    = 1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase beta-2 monomer, Human
}}

'''Phosphoinositide phospholipase C''' ('''PLC''', EC 3.1.4.11, '''triphosphoinositide phosphodiesterase, phosphoinositidase C, 1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase, monophosphatidylinositol phosphodiesterase, phosphatidylinositol phospholipase C, PI-PLC, 1-phosphatidyl-<small>D</small>-''myo''-inositol-4,5-bisphosphate inositoltrisphosphohydrolase'''; systematic name '''1-phosphatidyl-1<small>D</small>-''myo''-inositol-4,5-bisphosphate inositoltrisphosphohydrolase''') is a family of eukaryotic intracellular enzymes  that play an important role in signal transduction processes.<ref name="PUB00000624">{{cite journal |vauthors =Meldrum E, Parker PJ, Carozzi A |title=The PtdIns-PLC superfamily and signal transduction |journal=Biochim. Biophys. Acta |volume=1092 |issue=1 |pages=49–71 |year=1991 |pmid=1849017 |doi=10.1016/0167-4889(91)90177-Y}}</ref> These enzymes belong to a larger superfamily of [[Phospholipase C]].  Other families of phospholipase C enzymes have been identified in [[Zinc dependent phospholipase C|bacteria]] and [[Trypanosomatid|trypanosome]]s.  Phospholipases C are [[phosphodiesterases]].

Phospholipase Cs participate in [[phosphatidylinositol 4,5-bisphosphate]] (PIP<sub>2</sub>) metabolism and [[lipid signaling]] pathways in a calcium-dependent manner. At present, the family consists of six sub-families comprising a total of 13 separate [[isoforms]] that differ in their mode of activation, expression levels, catalytic regulation, cellular localization, membrane binding avidity and tissue distribution. All are capable of catalyzing the hydrolysis of PIP<sub>2</sub> into two important [[second messenger]] molecules, which go on to alter cell responses such as [[cell growth|proliferation]], [[differentiation (cellular)|differentiation]], [[apoptosis]], [[cytoskeleton]] remodeling, vesicular trafficking, [[ion channel]] [[Electrical conductance|conductance]], [[endocrine]] function and [[neurotransmission]].

==Reaction and catalytic mechanism==
All family members are capable of catalyzing the hydrolysis of PIP<sub>2</sub>, a [[phosphatidylinositol]] at the inner leaflet of the [[plasma membrane]] into the two second messengers, [[inositol trisphosphate]] (IP<sub>3</sub>) and [[diglyceride|diacylglycerol]] (DAG).

The [[chemical reaction]] may be expressed as:

:1-phosphatidyl-1<small>D</small>-''myo''-inositol 4,5-bisphosphate + H<sub>2</sub>O <math>\rightleftharpoons</math><br> 1<small>D</small>-''myo''-inositol 1,4,5-trisphosphate + diacylglycerol

PLCs catalyze the reaction in two sequential steps. The first reaction is a [[phosphotransferase]] step that involves an intramolecular attack between the [[hydroxyl]] group at the 2' position on the [[inositol]] ring and the adjacent [[phosphate]] group resulting in a cyclic IP<sub>3</sub> intermediate. At this point, DAG is generated. However, in the second [[phosphodiesterase]] step, the cyclic intermediate is held within the [[active site]] long enough to be attacked by a molecule of water, resulting in a final acyclic IP<sub>3</sub> product. It should be mentioned that bacterial forms of the enzyme, which contain only the catalytic [[lipase]] domain, produce cyclic intermediates exclusively, whereas the mammalian isoforms generate predominantly the acyclic product. However, it is possible to alter experimental conditions (e.g., temperature, pH) ''in vitro'' such that some mammalian isoforms will alter the degree to which they produce mixtures of cyclic/acyclic products along with DAG.{{Citation needed|date=October 2008}} This catalytic process is tightly regulated by reversible [[phosphorylation]] of different [[phosphoinositide]]s and their affinity for different regulatory proteins.<ref name="PUB00000014">{{cite journal |vauthors=Rhee SG, Choi KD |title=Multiple forms of phospholipase C isozymes and their activation mechanisms |journal=Adv. Second Messenger Phosphoprotein Res. |volume=26 |pages=35–61 |year=1992 |pmid=1419362}}</ref><ref name="PUB00002715">{{cite journal |vauthors =Rhee SG, Choi KD |title=Regulation of inositol phospholipid-specific phospholipase C isozymes |journal=J. Biol. Chem. |volume=267 |issue=18 |pages=12393–12396 |year=1992 |doi=10.1016/S0021-9258(18)42284-3 |pmid=1319994|doi-access=free }}</ref><ref name="PUB00005394">{{cite journal |vauthors=Sternweis PC, Smrcka AV |title=Regulation of phospholipase C by G proteins |journal=Trends Biochem. Sci. |volume=17 |issue=12 |pages=502–506 |year=1992 |pmid=1335185 |doi=10.1016/0968-0004(92)90340-F}}</ref>

==Cell location==
Phosphoinositide phospholipase C performs its catalytic function at the [[plasma membrane]] where the substrate PIP<sub>2</sub> is present. This membrane docking is mediated mostly by lipid-binding domains (e.g. [[PH domain]] and [[C2 domain]]) that display affinity for different [[phospholipid]] components of the plasma membrane. It is important to note that research has also discovered that, in addition to the plasma membrane, phosphoinositide phospholipase C also exists within other sub-cellular regions such as the [[cytoplasm]] and [[Cell nucleus|nucleus]] of the cell. At present, it is unclear exactly what the definitive roles for these enzymes in these cellular compartments are, particularly the nucleus.

==Function==
[[File:PIP2 cleavage to IP3 and DAG.jpg|right|thumb|235px|PIP2 cleavage to IP3 and DAG]]
Phospholipase C performs a catalytic mechanism, depleting PIP2 and generating [[inositol trisphosphate]] (IP<sub>3</sub>) and [[diglyceride|diacylglycerol]] (DAG).

Depletion of PIP2 inactivates numerous effector molecules in the plasma membrane, most notably PIP2 dependent channels and transporters responsible for setting the cell's membrane potential.<ref>{{cite journal|last1=Hansen|first1=SB|title=Lipid agonism: The PIP2 paradigm of ligand-gated ion channels.|journal=Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids|date=May 2015|volume=1851|issue=5|pages=620–8|doi=10.1016/j.bbalip.2015.01.011|pmid=25633344|pmc=4540326}}</ref>

The hydrolytic products also go on to modulate the activity of downstream proteins important for cellular signaling. IP3 is soluble, and diffuses through the cytoplasm and interacts with IP3 receptors on the [[endoplasmic reticulum]], causing the release of calcium and raising the level of intracellular [[calcium]].

''Further reading: [[Function of calcium in humans]]''

DAG remains within the inner leaflet of the plasma membrane due to its hydrophobic character, where it recruits [[protein kinase C]] (PKC), which becomes activated in conjunction with binding calcium ions. This results in a host of cellular responses through stimulation of calcium-sensitive proteins such as [[Calmodulin]].

''Further reading: [[Function of protein kinase C]]

{{Infobox protein family
| Symbol = PI-PLC-Y
| Name = PI-PLC-Y
| image = PDB 2fju EBI.jpg
| width = 
| caption = activated rac1 bound to its effector phospholipase c beta 2
| Pfam = PF00387
| Pfam_clan = CL0384 
| InterPro = IPR001711
| SMART = PLCYc
| PROSITE = PDOC50007
| MEROPS = 
| SCOP = 1qas
| TCDB = 
| OPM family = 126
| OPM protein = 2ptd
| CAZy = 
| CDD = 
}}

==Domain structure==
In terms of domain organization, all family members possess homologous X and Y catalytic domains in the form of a distorted Triose Phosphate Isomerase (TIM) barrel with a highly disordered, charged, and flexible intervening linker region. Likewise, all isoforms possess four [[EF hand]] domains, and a single C2 domain that flank the X and Y catalytic core. An N-terminal PH domain is present in every family except for the sperm-specific ζ isoform.

[[SH2 domain|SH2]] (phosphotyrosine binding) and [[SH3 domain|SH3]] (proline-rich-binding) domains are found only in the γ form (specifically within the linker region), and only the ε form contains both [[guanine nucleotide exchange factor]] (GEF) and RA ([[Ras subfamily|Ras]] Associating) domains. The β subfamily is distinguished from the others by the presence of a long C-terminal extension immediately downstream of the C2 domain, which is required for activation by G<sub>αq</sub> subunits, and which plays a role in plasma membrane binding and nuclear localization.

==Isoenzymes and activation==
The phospholipase C family consists of 13 isoenzymes split between six subfamilies, PLC-δ (1,3 & 4), -β(1-4), -γ([[PLCG1|1]],2), -ε, -ζ, and the recently discovered -η(1,2) isoform. Depending on the specific subfamily in question, activation can be highly variable. Activation by either [[Gq alpha subunit|G<sub>αq</sub>]] or G<sub>βγ</sub> [[G-protein]] subunits (making it part of a [[G protein-coupled receptor]] [[signal transduction]] pathway) or by [[transmembrane receptor]]s with intrinsic or associated [[tyrosine kinase]] activity has been reported. In addition, members of the Ras superfamily of small GTPases (namely the Ras and Rho subfamilies) have also been implicated. It should also be mentioned that all forms of phospholipase C require calcium for activation, many of them possessing multiple calcium contact sites in the catalytic region. The only isoform that is known to be inactive at basal intracellular calcium levels is the δ subfamily of enzymes suggesting that they function as calcium amplifiers that become activated downstream of other PLC family members.

===PLC-β===

PLC-β(1-4) (120-155kDa) are activated by G<sub>αq</sub> subunits through their [[C2 domain]] and long C-terminal extension. Gβγ subunits are known to activate the β2 and β3 isozymes only; however, this occurs through the PH domain and/or through interactions with the catalytic domain. The exact mechanism still requires further  investigation. The PH domain of β2 and β3 plays a dual role, much like PLC-δ1, by binding to the plasma membrane, as well as being a site of interaction for the catalytic activator.  However, PLC-β binds to the lipid surface independent of PIP<sub>2</sub> with all isozymes preferring phosphoinositol-3-phosphate or neutral membranes.

Members of the Rho GTPase family (e.g., Rac1, Rac2, Rac3, and [[cdc42]]) have been implicated in their activation by binding to an alternate site on the N-terminal PH domain followed by subsequent recruitment to the [[plasma membrane]]. A crystal structure of Rac1 bound to the PH domain of PLCβ2 has been solved.  Like PLC-δ1, many PLC-β isoforms (in particular, PLC-β1) have been found to take up residence in the nuclear compartment. A basic amino acid region within the enzyme's long C-terminal tail appears to function as a Nuclear Localization Signal for import into the nucleus. PLC-β1 seems to play unspecified roles in cellular proliferation and differentiation.

===PLC-γ===

'''PLC-γ''' (120-155kDa) is activated by receptor and non-receptor [[tyrosine kinases]] due to the presence of two SH2 and a single SH3 domain situated between a split PH domain within the linker region. Although this particular isoform does not contain classic nuclear export or localization sequences, it has been found within the nucleus of certain cell lines.{{Citation needed|date=July 2011}}  There are two main isoforms of PLCγ expressed in human specimens, PLC-γ1 and PLC-γ2.<ref name=Paul0808>{{cite book |last1=DeFranco |first1=Anthony |editor1-first=William |editor1-last=Paul |title=Fundamental Immunology |type=Book |edition=6th |year=2008 |publisher=Lippincott Williams & Wilkins |location=Philadelphia |isbn=978-0-7817-6519-0 |pages=270–288 |chapter=Chapter 8: B Lymphocyte Signaling Mechanisms and Activation}}</ref>

====PLC-''γ2''====
PLC-γ2 plays a major role in [[B cell receptor|BCR]] [[signal transduction]]. Absence of this enzyme in knockout specimens severely inhibits the development of B cells because the same signaling pathways necessary for antigen mediated B cell activation are necessary for B cell development from [[Haematopoiesis#The myeloid-based model|CLP]]s.<ref name=Paul0808/>

In B cell signaling, [[PI 3-kinase]] is recruited to the BCR early in the signal transduction pathway.  PI-3K phosphorylates PIP<sub>2</sub> ([[Phosphatidylinositol 4,5-bisphosphate]]) into PIP<sub>3</sub> ([[Phosphatidylinositol (3,4,5)-trisphosphate|Phosphatidylinositol 3,4,5-trisphosphate]]).  The increase in concentration of PIP<sub>3</sub> recruits PLC-γ2 to the BCR complex which binds to [[BLNK]] on the BCR scaffold and membrane PIP<sub>3</sub>. PLC-γ2 is then phosphorylated by [[Syk]] on one site and [[Bruton's tyrosine kinase|Btk]] on two sites. PLC-γ2 then competes with PI-3K for PIP<sub>2</sub> which it hydrolyzes into [[Inositol trisphosphate|IP<sub>3</sub>]] (inositol 1,4,5-trisphosphate), which ultimately raises intercellular calcium, and [[diacylglycerol]] (DAG), which activates portions of the [[Protein kinase C|PKC]] family. Because PLC-γ2 competes for PIP<sub>2</sub> with the original signaling molecule PI3K, it serves as a [[negative feedback]] mechanism.<ref name=Paul0808/>

===PLC-δ===
The PLC-δ subfamily consists of three family members, δ1, 2, and 3. PLC-δ1 (85kDa) is the most well understood of the three. The enzyme is activated by high calcium levels generated by other PLC family members, and therefore functions as a calcium amplifier within the cell. Binding of its substrate PIP<sub>2</sub> to the N-terminal [[PH domain]] is highly specific and functions to promote activation of the catalytic core. In addition, this specificity helps tether the enzyme tightly to the plasma membrane in order to access substrate through ionic interactions between the phosphate groups of PIP2 and charged residues in the PH domain. While the catalytic core does possess a weak affinity for PIP<sub>2</sub>, the C2 domain has been shown to mediate calcium-dependent phospholipid binding as well. In this model, the PH and C2 domains operate in concert as a "tether and fix" apparatus necessary for processive catalysis by the enzyme.

[[File:1djx_opm.png|thumb|250px|left|Phospholipase C-delta isoform 1. Blue plane shows hydrocarbon boundary of the [[lipid bilayer]]]]

PLC-δ1 also possesses a classical [[leucine]]-rich [[nuclear export signal]] (NES) in its [[EF hand]] motif, as well as a [[Nuclear localization signal]] within its linker region. These two elements combined allow PLC-δ1 to actively translocate into and out of the nucleus. However, its function in the nucleus remains unclear.

The widely expressed PLC-δ1 isoform is the best-characterized phospholipase family member, as it was the first to have high-resolution [[X-ray]] crystal structures available for analysis. In terms of domain architecture, all of the enzymes are built upon a common PLC-δ backbone, wherein each family displays similarities, as well as obvious distinctions, that contribute to unique regulatory properties within the cell. Because it is the only family found expressed in lower eukaryotic organisms such as [[yeast]] and [[slime molds]], it is considered the prototypical PLC isoform. The other family members more than likely evolved from PLC-δ as their domain architecture and mechanism of activation were expanded. Although a ''full'' crystal structure has not been obtained, high-resolution [[X-ray crystallography]] has yielded the molecular structure of the N-terminal PH domain complexed with its product IP3, as well as the remainder of the enzyme with the PH domain ablated. These structures have provided researchers with the necessary information to begin speculating about other family members such as PLCβ2.

===Other PLC families===
*'''PLC-ε''' (230-260kDa ) is activated by [[Ras subfamily|Ras]] and [[Rho family of GTPases|Rho]] GTPases.
*'''PLC-ζ''' (75kDa) is thought to play an important role in vertebrate [[fertilization]] by producing intracellular calcium oscillations important for the start of embryonic development. However, the mechanism of activation still remains unclear. This isoform is also capable of entering the early-formed [[pronucleus]] after fertilization, which seems to coincide with the cessation of calcium mobilization. It, like PLC-δ1 and PLC-β, possesses nuclear export and localization sequences.
*'''PLC-η''' has been implicated in neuronal functioning.

==Human proteins in this family ==
[[PLCB1]];     [[PLCB2]];     [[PLCB3]];     [[PLCB4]];     [[PLCD1]];     [[PLCD3]];     [[PLCD4]];     [[PLCE1]];
[[PLCG1]];     [[PLCG2]];     [[PLCH1]];     [[PLCH2]];     [[PLCL1]];     [[PLCL2]];     [[PLCZ1]]

==See also==
* [[Clostridium perfringens alpha toxin]]
* [[Lipid signaling]]
* [[PH domain]], found in some phospholipases C
* [[Phospholipase]]
* [[Zinc-dependent phospholipase C]], a different family of [[phospholipase C]]

==References==
{{reflist}}
* {{cite journal | vauthors = Downes CP, Michell RH | year = 1981 | title = The polyphosphoinositide phosphodiesterase of erythrocyte membranes | journal = Biochem. J.  | volume = 198 | pages = 133&ndash;40  | pmid = 6275838 | issue = 1 | doi = 10.1042/bj1980133 | pmc = 1163219 }}
* {{cite journal |author1 = Thompson W |author2 =Dawson RMC | year = 1964 | title = The triphosphoinositide phosphodiesterase of brain tissue | journal = Biochem. J.  | volume = 91 | pages = 237&ndash;243 | pmid=4284484 | issue = 2 | pmc = 1202878 | doi=10.1042/bj0910237}}
* {{cite journal | vauthors = Rhee SG, Bae YS | year = 1997 | title = Regulation of phosphoinositide-specific phospholipase C isozymes | journal = J. Biol. Chem.  | volume = 272 | pages = 15045&ndash;8  | pmid = 9182519 | doi = 10.1074/jbc.272.24.15045 | issue = 24 | doi-access = free }}
* {{MeshName|Phospholipase+C}}

{{Intracellular signaling peptides and proteins}}
{{Lipid signaling}}
{{Esterases}}
{{Enzymes}}
{{Portal bar|Biology|border=no}}

[[Category:EC 3.1.4]]
[[Category:Peripheral membrane proteins]]
[[Category:Enzymes of known structure]]
[[Category:Signal transduction]]
[[Category:Protein families]]
[[Category:Enzymes]]
[[Category:Calcium enzymes]]
[[Category:Hydrolases]]
[[Category:Calcium signaling]]
[[Category:Cell signaling]]
[[Category:G protein-coupled receptors]]
[[Category:Cell biology]]

[[de:Phospholipase C]]
[[es:Fosfolipasa C]]
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[[he:פוספוליפאז C]]
[[ru:Фосфолипаза C]]