Protein phospholipase

MS Baida, GL Mariscal, RG Contreras, MM Silva, TME Marquez, GJA Sainz, M Cereijido. (1991). Assembly and sealing of tight junctions Possible participation of G-proteins, phospholipase C, protein kinase C, and calmodulin. J Membr Biol 122 193-202. 

Inositol triphosphate (IP3)-gated channels are also associated with membrane-bound receptors for hormones and neurotransmitters. In this case, binding of a given substance to its receptor causes activation of another membrane-bound protein, phospholipase C. This enzyme promotes hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) to IP3. The IP3 then diffuses to the sarcoplasmic reticulum and opens its calcium channels to release Ca++ ions from this intracellular storage site. 

Voltage-dependent Na+, K+, Ca2+ channels Ca2+-dependent potassium channels Enzymes and other proteins involved in the regulation of second messengers G proteins Phospholipases Adenylyl cyclases Guanylyl cyclases Phosphodiesterases IP3 receptor Protein kinases... 

The substrates of the MAP kinase pathway are very diverse and include both cytosolic and nuclear localized proteins. Phospholipase A2 and transcription factors of the Ets family are well characterized substrates of the ERK pathway. Phosphorylation of a Ser residue of phospholipase A2 by ERK proteins leads to activation of the lipase activity. Consequently, there is an increase in release of arachidonic acid and of lyso-phospholipids, which can act immediately as diffusible signal molecules or may represent first stages in the formation of second messenger molecules. 

It is fitting to introduce the phospholipase pathways here because they are controlled by small G proteins. Phospholipases C and D (PLC, PLD) are controlled by Rho/Arf and Cdc42 and Src tyrosine kinases participate in the control of PLD. 36 Phospholipases form potent second messengers, such as inositol trisphosphate (IP3) and diacylglycerol (DAG). In Fig. 4.6a the reactions catalysed by phospholipases C and D and the connections between phospholipases C and D are summarized, and in Fig. 4.6b the regulatory pathways that activate phospholipase D synergistically are summarized.37... 

Calcium ions constitute only one of several participants in the pathway of "calcium signaling." The role of cytosolic Ca ions, in this pathway, is to rise suddenly in concentration over a course of a few seconds or minutes, and then to fall back to basal levels. The following commentary concerns events that occur before, during, and after the burst of calcium ions. An extracellular stimulant, such as a hormone, binds to a receptor in the cell membrane. The event of binding provokes the activation of a membrane-bound protein, phospholipase C,... 

Established calcium-requiring enzymes and proteins are listed in Table 10.15. The calcium metalloenzymes include various hydnjlytic enzymes. These hydrolases act on polysaccharides, phospholipids, and proteins. Phospholipase C was discussed earlier. Even though phospholipase C requires calcium and is involved in the Ca signaling pathway, it is believed that the changes in cytosolic Ca levels that occur with Ca signaling do not influence the activity of this enzyme. 

The binding of many different hormones, ncumtransmil-ters. and growth factors to the cell surface results in activation of the polyphosphoinositide receptor. system.Binding to the specific receptor activates the enzyme phospholipase C through the intermediacy of a G protein. Phospholipase C converts pho.sphatidylinositol 4 5-bisphosphatc (PIP-) into IP3 and DAG. IPj releases calcium ion. which in turn all ccls many cellular responses in the target eells. 

In the PIP2-Ca signal transduction system, the signal is transferred from the epinephrine receptor to membrane-bound phospholipase C by G proteins. Phospholipase C hydrolyzes PIP2 to form diacylglycerol (DAG) and inositol trisphos-phate (IP3). IP3 stimulates the release of Ca from the endoplasmic reticulum. Ca and DAG activate protein kinase C. The amount of calcium bound to one of the calcium-binding proteins, calmodulin, is also increased. 

FIGURE 24.12 The PIP2 second-messenger scheme. When a hormone binds to a receptor, it activates phospholipase C, in a process mediated by a G protein. Phospholipase C hydrolyzes PIPj to IP3 and DAG. IP3 stimulates the release of Ca from intracellular reservoirs in the ER. A complex formed between Ca and the calcium-binding protein calmodulin activates a cytosolic protein kinase for phosphorylation of a target enzyme. DAG remains bound to the plasma membrane, where it activates the membrane-bound protein kinase C (PKC). PKC is involved in the phosphorylation-channel proteins that control the flow of Ca + in and out of the cell. Ca from extracellular sources can produce sustained responses even when the supply of Ca + in intracellular reservoirs is exhausted. 

Finally, both cloned [alpha(lG), alpha(lH) and alpha(ll) subunits] and native T-type Ca + chaimels are blocked by AEA at submicromolar concentrations, independently from the activation of CBj/CBj receptors, G-proteins, phospholipases, and protein kinase (PK) pathways. Block of AEA transport into cells via the AEA membrane transporter (AMT) prevented T-current inhibition, snggesting that AEA acts at an intracellular site (Chemin et al., 2001). 

King, T.P., Kochoumian, L. and Joslyn, A., 1984, Wasp venom proteins Phospholipase A1 and B. Arch. Biochem. Biophys. 230 1-12. 

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