Lipid Raft and Signal Transduction Tao Chen

Lipid Raft A cholesterol-enriched microdomain in cell membrane. A liquid-ordered phase dispersed in a liquid disordered matrix of cell membrane, which is thought to minimize the free energy between the two phases. Can include and exclude proteins to variable extents. Resistent to non-ionic detergents, such as Triton X-100 or Brij-98 at low temperatures (e.g., 4 ℃ ). Also called DRM, DIG, DIC, GPI domain, Glycosphingolipid signaling domain, caveolae-like domain, microdomain, LDM, lipid-ordered domain, DIM, GEM and TIFF.

A Intracellular space or cytosol B Extracellular space or vesicle/Golgi apparatus lumen (1)Non-raft membrane (2)Lipid raft (3)Lipid raft associated transmembrane protein (4)Non-raft membrane protein (5)Glycosylation modifications (6)GPI-anchored protein (7)Cholesterol (8)Glycolipid

Techniques to Identify Lipid Raft:

Signal Transduction Signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another. Ligand bind to receptor Receptor change its conformation Enzyme bind to receptor Enzyme is activated Red: chemical messenger Yellow: transmembrane protein Purple: cell membrane Green: enzyme

Through the connection of raft proteins and interacting proteins to form a complex, resulting in the activation of signalling cascade.

(a) Receptors associated at steady state with lipid rafts could be activated through ligand binding (b) Individual receptors with weak raft affinity could oligomerize on ligand binding, and this would lead to an increased residency time in rafts. (1) Single Rafts Model

Activated receptors could recruit cross linking proteins that bind to proteins in other rafts, and this would result in raft coalescence. (2) Clustered Rafts Model

Clustering of lipid raft can occur both extracellularly, within the membrane, and in the cytosol. It can also occur through GPI anchored proteins, either as a primary or co-stimulatory response. These two models are not mutually exclusive.

T-Cell Antigen Receptor Signalling The T cell receptor or TCR is a heterodimer consisting of an alpha and beta chain. Alpha, beta heterdimer associate with CD3 complex and ζ homodimer which contain cytoplasmic ITAM motifs (immune receptor tyrosine-based activation motifs).

Phosphorylated ITAMs act as a membrane docking site for ZAP-70 ZAP-70 activate other proteins such as LAT, a raft- associated adaptor, through cross linking, LAT can recruit other proteins into the raft and further amplify the signal. Cytosolic kinase Csk bind to raft-associated protein CBP and then inactivate the Src-family kinases through phosphorylation Ligand-induced receptor dimerization of TCR/CD3 Phosphorylation of the receptor ’ s ITAMs by Src-family protein tyrosine kinases

Open Issues: Raft composition Whether more than one kind of raft exists on the cell surface of different cell types. Understanding the nature of individual rafts. How single rafts are crosslinked to form clustered rafts during signal transduction.

References Kai Simons and Derek Toomre, Lipid Rafts and Signal Transduction, Nature Reviews-Molecular Cell Biology, 2000, 1, Brown D A, London E, Functions of Lipid Rafts in Biological Membranes, Annu. Rev. Cell. Dev. Biol, 1998, 14, Schroeder R, London E, Brown D, Interactions between saturated acyl chains confer detergent resistent on lipids and glycosylphosphatidylinositol(GPI)-anchored proteins: GPI-anchored proteins in liposomes and cells show similar behavior, Proc. Natl Acad. Sci. USA, 1994, 91, Zhang W, Trible R P, Samelson L E, LAT palmitoylation: its essential role in membrane microdomain targeting and tyrosine phosphorylation during T-cell activation, immunity, 1998, 9,