Membrane+structure+and+function_part+2.pdf

Membrane Structure Part 2 (BIO4212) 1st Term, A.Y. 2024-2025  Some lipids form domains in membranes - & ol proteir binded Lipid rafts – phase segregations of lipids with concentrations of membrane proteins where protein-protein,...

Membrane Structure Part 2 (BIO4212) 1st Term, A.Y. 2024-2025  Some lipids form domains in membranes - & ol proteir binded Lipid rafts – phase segregations of lipids with concentrations of membrane proteins where protein-protein, protein-lipid and lipid-lipid interactions occur. Entire lipid raft domain may be involved in transport of membrane vesicles or conversion of extracellular signal to intracellular signal during signal transduction.  Lipid Rafts in Signal Transduction  B cell receptors dimerize when they interact with an antigen in a confined area of the membrane which is an example of lipid raft.  Followed by signal transduction leading to antibody production.  Lipid raft is where proteins interact with other proteins and lipids. 1 Signal transduction rospholipid id. 2. P ↳ sphingo ↳ cholestero Lipid Rafts in Endocytosis Lipid raft involved in endocytosis either by:  caveola formation g  Transport vesicle formation Lipid rafts contains G-protein coupled receptors. s as ch a Involved in the formation of vesicles for the transport of neurotransmitter from the extracellular matrix &  Lipid Droplets are enclosed by phospholipid monolayer ER in released des fets g · veg Agg store excessa and ↑ Lipid droplets are unique organelles that store excess neutral lipids (to be used as building blocks for membrane synthesis or food) Enclosed by monolayer of phospholipids with associated proteins (some are enzymes involved in lipid metabolism) Formed by budding and pinching and budding off from ER Found in adipocytes and other cells exposed to high concentrations of fatty acids Lipid compositions of the 2 monolayers in cell membranes are different Glycolipid phosphatidylcholine Sphingomyelin Human erythrocyte Plasma Membrane phospholipid Pfdser , PtoEtu ↳ flipase ↑ L extra to mediated intr a ↳ Scramblase phosphatidylserine ↳ non-specific phosphatidylethanolamine to extra ↳ intra Outer monolayer contains phospholipids with choline heads (phosphatidylcholine and sphingomyelin) Inner monolayer contains mostly terminal primary amino group (phosphatidylethanolamine and phosphatidylserine) Difference in net charges of the lipid molecules (mostly negatively charged on the cytoplasmic side) Many cytosolic proteins bind to lipid heads on the inner monolayer (Protein Kinase C (PKC), in Phosphatidylinositol (PI), Phosphoinositide 3-kinase (PI 3-Kinase), phospholipase C X expressed wells highly minschendric cancer ⑫ i functio a sa e dun ⑭ sbind , de  Lipid compositions of the 2 monolayers in cell membranes are different Glycolipid phosphatidylcholine Sphingomyelin Human erythrocyte Plasma Membrane phosphatidylserine phosphatidylethanolamine ↑ nag translocatee Animal cells distinguish apoptotic cells by translocation of phophatidylserine from inner to outer monolayer serving as a signal for macrophages for phagocytosis. Apoptosis results to the activation of scramblase, an enzyme that translocates phospholipids nonspecifically while phospholipid translocator is deactivated (transport of lipids from outer to inner side. Membrane Proteins  Integral membrane proteins function as receptors that bind ligands, channels or transporters to move ions/solutes across the membrane. Amphipathic  Peripheral membrane proteins – non-covalent interactions with polar heads of lipids. Cytochrome C  Lipid-anchored protein – covalently linked to lipids Glycosylphosphatidylinositol (GPI)-anchored proteins Fatty acid Prenyl group – long chain of hydrocarbon built from 5-C isoprenoid units. binds * sa glycolipids Copyright ©2020 John Wiley & Sons, Inc. Studying the Structure and Properties of Integral Membrane Proteins  It is very difficult to obtain crystals of integral membrane proteins for X-ray crystallography  Obstacles:  Present in low numbers per cell  Unstable in the detergent-containing solution  Prone to aggregation  Modified by glycosylation  Homology modeling is used to learn about the structure and activity of members of a An integral protein as it resides within the protein family. plasma membrane Copyright ©2020 John Wiley & Sons, Inc. Transmembrane proteins have -helices The transmembrane domain of integral fold membrane proteins often contains -helical strey A conformation -helix is often composed of mostly of hydrophobic amino acids found along the hydrophobic region of the bilipid layer. Hydrophobic amino acids from hydrogen Pinteract bonds with one another w hydrogen  Identifying transmembrane domains Summarizes the hydropathy index (number of hydrophobic or hydrophilic properties of aa) of a peptide or protein. Single-pass transmembrane proteins – with one -helix traversing the bilipid layer Multipass transmembrane - proteins – with several -helices traversing the bilipid layer hydropathy -helix contains ~20-30 energy required > - non-polar to moleme transfer hydrophobic amino acids to water > - non spontaneous - AG : non-spontaneous - AG : Spontaneous Aquaporin Channel assumed no energ Passage of water molecules in cell membranes Exist as a tetramer (4 monomers) where each monomer is composed of 6 𝘢-helices Two short 𝘢-helices spans halfway through the lipid bilayer (confer substrate specificity) With NPA motif (Asparagine-Proline-Alanine) · polar polarang Transmembrane -helices interact with one another more thergeticala Dimerization of 2 monomers of Protein-protein (helix-helix) interactions of Glycophorin A involves seven- multipass transmembrane proteins are residue-motif along the helix-helix crucial in channel proteins and transporter interaction proteins dectrostatic protan proter a was β Barrels Form Large Channels Multipass transmembrane proteins form β barrels Rigid proteins Outer membranes of bacteria, mitochondria and chloroplasts non-polmyof Porins, enzymes, receptors, anchor for integral proteins Polar amino acid side chains: lining of channels  Membrane Proteins Associated with the Lipid Bilayer Peripheral Proteins  located entirely outside of bilayer on either the extracellular or cytosolic side  Attached to the cytosolic side by amphipathic α helix (4)  Covalent attachment of fatty acid chains into cytosolic monolayer (5)  associated with membrane surface by non-covalent bonds (ionic, protein – protein interaction) (7,8)  easily solubilized by aqueous salt solution and extreme pH Lipid Anchors control membrane localization of some signaling proteins magbind Switch-like - may ud Some membrane proteins are transiently attached to membranes as they transition from soluble to insoluble form (e.g. Rab proteins) -charge Src family of cytoplasmic Ras family of small peripheral proteins tyrosine kinases are GTPases are attached myristoylated via prenyl or palmitic acid molecules  Determining Membrane packed Protein Sidedness rightly & bigger top [] ↑ travel nag smaller pababa bottom (2 transmembrane proteins ↑ - Ge electrophoresis top() 30] ↓ 7 to attracted positive bottom ()  Membrane Proteins Glycosylation small clusters of sugar / Glycoproteins: Oligosaccharides covalently linked to proteins Proteoglycans: long polysaccharide chains Location: extracellular surface of the bilayer ( (glycocalyx: protective layer - carbohydrate coat); noncytosolic side of membrane Functions: Cell protection against damage; prevent unwanted cell- cell interactions, maintains cell distancing; cell recognition, cell- cell adhesion Carbohydrates form a cell coat (Glycocalyx) Glycocalyx functions for protection via covalent interactions with proteins (glycoproteins) of with lipids (oligolipids). Membrane proteins can be solubilized and purified Detergents are small amphiphilic molecules that can be used to solubilize membrane proteins At low concentrations, detergents are monomeric but once they reach critical micelle concentration (CMC) they form micelles SDS solubilize membrane proteins allowing analysis in SDS- PAGE Solubilizing membrane with mild non-ionic detergents Detergents competes with the hydrophobic interactions of proteins with lipids forming water-soluble protein-lipid- detergent complexes and lipid-detergent micelles Purification of membrane proteins Mild nonionic detergents (Tween 20, Triton X, digitonin) can solubilize membrane proteins but they don’t unfold the proteins Useful to isolate functional proteins to study their roles in biological systems (transporters, ion channels, signaling receptors, etc). Purified membrane proteins may be incorporated in phospholipid vesicles to study their functions. Membrane proteins can be reconstituted in nanodiscs Nanodiscs are small, uniformly sized patches of membrane that surrounded by belt of protein. Protein belt is a high-density lipoproteins (HDL) Proteins can be studied in their native states accessible on both sides of the lipid bilayer May be studied using Single- Particle Electron Microscopy Some membrane proteins traverses the bilipid layer several times Bacteriorhodopsin is a light-activated H+ pump involved in the transfer of H+ O out the archeal cell Identified by Electron Crystallography (combination of electrom microscopy and electron diffraction analysis) and X- ray Crystallography. Composed of 7 -helices of transmembrane domain and retinal (light absorbing group) Process of H+ transfer will drive production of ATP Cells restrict certain proteins at specific domains Lateral diffusion of proteins are restricted to specific domains of a cell where their functions are needed. In epithelial cells, enzymes and transport proteins are restricted to the apical surface while proteins involved in cell-cell communications are limited to the lateral plasma membrane and proteins involved in attachment are restricted to basal plasma membrane Ways of restriction lateral diffusion of proteins Membrane proteins can self-assemble to form large aggregates of proteins Tethering by interactions with other extracellular macromolecules Tethering by interactions with other intracellular macromolecules Interaction with other proteins in another cell  Membrane Domains and Cell Polarity Intestinal epithelial cells: differences in the proteins of apical, lateral & basal membranes Highly differentiated sperm have a head, midpiece, and tail that is covered by a continuous membrane.  Cells Can Confine Proteins and Lipids to Specific Domains Within a Membrane Protein self-assembly into large aggregates Protein tethering by macromolecule interactions outside and inside the cell Cell surface protein interactions The Cortical Cytoskeleton Gives Membranes Mechanical Strength and Restricts Membrane Protein Diffusion Erythrocyte Membrane Skeleton  Spectrin: cortical cytoskeleton Heterodimer attached to the membrane surface by noncovalent bonds to ankyrin (links spectrin to the cytosolic face of band 3) linked to other cytoplasmic proteins, such as actin and tropomyosin, which maintains the integrity of the membrane; elasticity and pliability > - spectric O  Membrane-bending Proteins Deform Bilayers Insertion of hydrophobic protein domains increases surface area Attachment of lipid anchors Binding of curved proteins bin to lipid head groups Clustering of membrane lipids Summary ability ng and cell a function ↑ essere Proteins are responsible for most of the membrane functions Transmembrane proteins are diverse which includes single pass and multi- pass membrane proteins Some proteins are limited to either leaflets of the bilipid membrane. Variety of attachment methods of proteins to the bilipid layer exist in cells Oligosaccharides are attached to membrane proteins are responsible for a variety of functions. Proteins diffuse on the surface of cell membranes. Lateral movement of proteins are restricted by cytoskeletal proteins and other interactions with macromolecules Membrane-bending proteins are responsible for the 3 dimensional shape of cells