What are the Proteins in the Lipid Valve Structure of the Cell Membrane?
There are usually many proteins in the lipid valve structure of the cell membrane, which can be roughly divided into three categories:
(1) Integral proteins: This type of protein is partially or completely embedded in the cell membrane or on both the inner and outer sides, and is bound to the plasma membrane by the interaction of non-polar amino acids with the non-polar hydrophobic region of the lipid bilayer.
(2) Anchored proteins: They can be divided into two categories. One is the protein linked to glycophosphatidylinositol (GPI). GPI is located in the outer leaflet of the cell membrane. Treating cells with phospholipase C (which can recognize phospholipids containing inositol) can release the bound protein. Many cell surface receptors, enzymes, cell adhesion molecules and PrPC that causes scrapie are such proteins. Another type of lipid-anchored protein is bound to a long hydrocarbon chain inserted into the inner leaflet of the plasma membrane.
(3) Peripheral proteins: They are bound to the hydrophilic part of protein molecules or lipid molecules on the membrane surface by ionic bonds or other weaker bonds. Therefore, they can be separated from the membrane by changing the ionic strength of the solution or even increasing the temperature.
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What is the Structure of Glycosylphosphatidylinositolated Proteins
The structure of glycosylphosphatidylinositolated proteins (GPI-anchored proteins) is divided into three parts
The first is the protein: the C-terminal group is connected to the GPI anchor, usually Gly, Ala, Ser, Cys, Asp, Asn. In fungal cells, the N-terminus and C-terminus of GPI-anchored proteins have signal peptides. The C-terminus can effectively induce it to bind to the GPI anchor, so it is called the anchor signal peptide of the GPI-anchored protein. It is generally a stretch of uncharged hydrophobic aminocaproic acid, and there is a GPI binding site ω point near the C-terminus 9-10 amino acids. The middle of the protein is composed of functional groups and tryptophan/serine-rich structural regions. The N-terminal signal peptide, the C-terminal hydrophobic amino acids, and the ω point near the C-terminus are the conserved structural domains of the GPI-anchored protein.
The second part is the GPI anchor, which includes a phosphoethanolamine bridge, a sugar chain, and inositol phosphate. The sugar chain is linked to the 6th position of inositol phosphate, and the common tetrasaccharide structure is Man-Man-Man-GlcN.
The third part is a long fatty chain inserted into the lipid bilayer, which is generally acylglycerol or glycerol ether, and some are replaced by ceramide. The GPI anchor protein is connected to the phosphoethanolamine connected to the trimannosyl-nonacetylated glucosamine (Man3-GlcN) core through a phosphodiester bond at its carboxyl terminus. The reducing end of GlcN is connected to phosphatidylinositol (PI) through another phosphodiester bond.
What Do GPI-anchored Proteins Bind to?
The biosynthesis of GPI-anchored proteins is a type of post-translational modification of proteins that is widely present in all eukaryotic organisms, and the biochemical reactions involved in this modification process are conserved, including three main steps:
(1) Synthesis of GPI molecules on the endoplasmic reticulum membrane;
(2) The carboxyl-terminal (C-terminal) signal peptide of the GPI-AP precursor protein (ProGPI-AP) is removed and covalently linked to the GPI molecule to form GPI-AP;
(3) GPI-AP undergoes further processing and maturation and is transported to the cell membrane through the Golgi apparatus. The GPI amidotransferase complex is the key catalytic enzyme in the second step of GPI-anchored protein biosynthesis and consists of five subunits, including PIGK, PIGU, PIGT, PIGS and GPAA1.
What are the Functions of Glycosylphosphatidylinositol?
In the human body, at least 150 proteins have been identified as GPI-anchored proteins, which are widely distributed in various types of cells; these GPI-anchored proteins are involved in many biological processes, including membrane-associated enzyme activity (PLAP, Phospholipase B, Aspartic proteinase, etc.), cell signal transduction (CD55, CD59 and calcium channel regulatory subunit a2d1, etc.), cell adhesion (OPCML, CD56, etc.) and antigen presentation (Thy-1, CD14, CD52, etc.).
