Lecture 1.2 Proteins of Cell Membrane PDF

Membranes and receptors module Session 1 Lecture 1.2 Proteins of cell membrane Dr.Asmaa hamza References 1. Introduction to General, Organic and Biochemistry Frederick A. Bettelheim 2. Alberts, et al., Molecular Biology of the Cell: Fourth Edition, New York: Garland Science, 2002 3....

Membranes and receptors module Session 1 Lecture 1.2 Proteins of cell membrane Dr.Asmaa hamza References 1. Introduction to General, Organic and Biochemistry Frederick A. Bettelheim 2. Alberts, et al., Molecular Biology of the Cell: Fourth Edition, New York: Garland Science, 2002 3. Lehninger principles of biochemistry. Albert L. Lehninger Objectives: To consider - The distribution and role of proteins in membrane structure - The importance of an asymmetric distribution of membrane proteins - The structure of the erythrocyte cytoskeleton. Objective number 1: The distribution and role of proteins in membrane structure  Lipid mosaic theory of membrane structure (Singer – Nicholson model) Biological membranes are composed of a lipid bilayer with associated membrane proteins which may be Peripheral: Bound to the surface of membranes by electrostatic and hydrogen bond interactions. These proteins can be removed by changes in pH or ionic strength. - Integral: Deeply embedded. Interact extensively with the hydrophobic regions of the lipid bilayer , cannot be removed by changes of pH or ionic strength but require detergent or organic solvents. Objective 2 The importance of an asymmetric distribution of membrane proteins Asymmetrical orientation of membrane proteins Asymmetrical orientation of proteins in biological membranes is important for function e.g. a receptor for a hydrophilic extracellular messenger molecule, such as insulin, must have its recognition site directed towards the extracellular space to be able to function Objective number 3 - the structure of the erythrocyte cytoskeleton. The erythrocyte membrane ( amodel of plasma membrane) Erythrocyte ghosts can be prepared by osmotic hemolysis to release cytoplasmic components. Analysis of ghost membranes by gel electrophoresis reveals over 10 major proteins. RBC membrane proteins How we can distinguish between peripheral proteins and integral proteins under gel electrophoresis in Erythrocyte ghosts? Most of these proteins are released when ghost membranes are treated with high ionic strength medium or by changing the pH and are, thus, peripheral proteins. These peripheral proteins must be located on the cytoplasmic face since they are susceptible to proteolysis only when the cytoplasmic face of the membrane is accessible.  The major ones have been numbered 1, 2, 3, 4.1, 4.2, 5, 6 and 7 etc..  Protein bands 3 and 7 can only be dissociated from the red cell membrane by detergents and are, thus, integral proteins. Both proteins contain covalently attached carbohydrate units and are, thus, glycoproteins.  The highly hydrophilic nature of the extracellular carbohydrate groups acts to lock the orientation of the protein in the membrane by preventing flip-flop rotation. Specific carbohydrate groups on membrane proteins may be important for cellular recognition to allow tissues to form and in immune recognition. Cytoskeleton (Membrane skeleton) A cellular protein structure like a skeleton, attached to the cell membrane of the cytoplasmic side and found in the cytoplasm. The erythrocyte cytoskeleton is a network of spectrin and actin molecules Spectrin is a long, floppy rod-like molecule. Alfa and beta subunits wind together to form an antiparallel heterodimer and two heterodimers then form a head-to-head association to form a heterotetramer of Alfa2beta2  These rods are cross linked into networks by short actin protofilaments (~14 actin monomers), and band 4.1 and adducin molecules which form interactions towards the ends of the spectrin rods. The spectrin-actin network is attached to the membrane through adapter proteins. Ankyrin (band 4.9) and band 4.1 link spectrin and band 3 protein and glycophorin A,respectively. Attachment of integral membrane proteins to the cytoskeleton restricts the lateral mobility of the membrane protein. HAEMOLYTIC ANAEMIAS TheRBC cytoskeleton is a very important structure in maintaining the deformability necessary for erythrocytes to make their passage through capillary beds without lysis. 1- Hereditary Spherocytosis spectrin levels may be depleted by 40 - 50%. The cells round up and become much less resistant to lysis during passage through the capillaries and are cleared by the spleen. The shortened in vivo survival of red blood cells and the inability of the bone marrow to compensate for their reduced life span lead to hemolytic anaemia 2- Hereditary Elliptocytosis a common defect is a spectrin molecule that is unable to form heterotetramers resulting in fragile elliptoid cells. Even simple treatment with cytochalasin drugs, which cap the growing end of polymerizing actin filaments, can alter the deformability of the erythrocyte.