Phosphoinositides protein lipid-binding domains

New developments in immobilization surfaces have lead to the use of SPR biosensors to monitor protein interactions with lipid surfaces and membrane-associated proteins. Commercially available (BIACORE) hydrophobic and lipophilic sensor surfaces have been designed to create stable membrane surfaces. It has been shown that the hydrophobic sensor surface can be used to form a lipid monolayer (Evans and MacKenzie, 1999). This monolayer surface can be used to monitor protein-lipid interactions. For example, a biosensor was used to examine binding of Src homology 2 domain to phosphoinositides within phospholipid bilayers (Surdo et al., 1999). In addition, a lipophilic sensor surface can be used to capture liposomes and form a lipid bilayer resembling a biological membrane. 

Janmey, P.A., Xian, W. and Flanagan, L.A., 1999, Controlling cytoskeleton structure by phosphoinositide-protein interactions phosphoinositide binding protein domains and effects of lipid packing. Chem. Phys. Lipids 101 93-107. 

Figure 14.20 Insulin signaling. The binding of insulin results in the cross-phosphorylation and activation of the insulin receptor. Phosphorylated sites on the receptor act as binding sites for [insulini receptor substrates such as IRS-1. The lipid kinase phosphoinositide 3-kinase binds to phosphorylated sites on IRS-1 through its regulatory domain, then converts PIPj into PIPv Binding to PiP activates PIP3-dependent protein kinase, which phosphorylates and activates kinases such as Aktl. Activated Aktl can then diffuse throughout the cell to continue the signal-transduction palhway.

Figure 14.21 The modular structure of Insulin receptor substrates IRS-1 and IRS-2 This schematic view represents the amino acid sequence common to IRS-1 and lRS-2. Each protein contains a pleckstrin homology domain (which binds phosphoinositide lipids), a phospbotyrosine-binding domain, and four sequences that approximate Tyr-X-X-Met (YXXM). The latter are phosphorylated by the insulin receptor tyrosine kinase.

Pleckstrin homology domain (PH-domain) was first identified at the amino and carboxyl termini of a haematopoietic protein called pleckstrin. PH-domain, a protein region of approximately 120 amino acids, by binding to phosphatidylinositol lipids of the biological membranes induces the translocation of the PH-domain containing protein to membrane compartment. Various PH-domains possess specificities for phosphoinositides phosphorylated at different sites within the inositol ring. 

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