Membrane protein - wikipedia Membrane protein Crystal structure of Potassium channel Kv1. 2 / 2.1 Chimera. Calculated hydrocarbon boundaries of the lipid bilayer are indicated by red and blue dots. Membrane proteins are proteins that interact with, or are part of, biological membranes. They include integral membrane proteins that are permanently anchored or part of the membrane and peripheral membrane proteins that are only temporarily attached to the lipid bilayer or to other integral proteins. The integral membrane proteins are classified as transmembrane proteins that span across the membrane and integral monotopic proteins that are attached to only one side of the membrane. Membrane proteins are a common type of proteins along with soluble globular proteins, fibrous proteins, and disordered proteins. They are targets of over 50 % of all modern medicinal drugs. It is estimated that 20 -- 30 % of all genes in most genomes encode membrane proteins. Compared to other classes of proteins, the determination of membrane protein structure s has remained a challenge in large part due to the difficulty in establishing experimental conditions where the correct conformation of the protein in isolation from its native environment is preserved. Contents 1 Function 1.1 Integral membrane proteins 1.2 Peripheral membrane proteins 1.3 Polypeptide toxins 2 In genomes 3 See also 4 References 5 External links 5.1 Organizations 5.2 Membrane protein databases Function Membrane proteins perform a variety of functions vital to the survival of organisms : Membrane receptor proteins relay signals between the cell 's internal and external environments. Transport proteins move molecules and ions across the membrane. They can be categorized according to the Transporter Classification database. Membrane enzymes may have many activities, such as oxidoreductase, transferase or hydrolase. Cell adhesion molecules allow cells to identify each other and interact. For example, proteins involved in immune response. Integral membrane proteins Schematic representation of transmembrane proteins : 1. a single transmembrane α - helix ( bitopic membrane protein ) 2. a polytopic transmembrane α - helical protein 3. a polytopic transmembrane β - sheet protein The membrane is represented in light - brown. Main articles : Integral membrane protein and Transmembrane protein Integral membrane proteins are permanently attached to the membrane. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents. One such example of this type of protein which has not been functionally characterized yet is SMIM23. They can be classified according to their relationship with the bilayer : Integral polytopic proteins, also known as `` transmembrane proteins '', are integral membrane proteins that span across the membrane at least once. These proteins may have different transmembrane topology. These proteins have one of two structural architectures : helix bundle proteins, which are present in all types of biological membranes ; beta barrel proteins, which are found only in outer membranes of Gram - negative bacteria, and outer membranes of mitochondria and chloroplasts. Integral monotopic proteins are integral membrane proteins that are attached to only one side of the membrane and do not span the whole way across. Peripheral membrane proteins Schematic representation of the different types of interaction between monotopic membrane proteins and the cell membrane : 1. interaction by an amphipathic α - helix parallel to the membrane plane ( in - plane membrane helix ) 2. interaction by a hydrophobic loop 3. interaction by a covalently bound membrane lipid ( lipidation ) 4. electrostatic or ionic interactions with membrane lipids ( e.g. through a calcium ion ) Main article : Peripheral membrane protein Peripheral membrane proteins are temporarily attached either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations. Integral and peripheral proteins may be post-translationally modified, with added fatty acid or prenyl chains, or GPI ( glycosylphosphatidylinositol ), which may be anchored in the lipid bilayer. Polypeptide toxins Main article : Pore - forming toxin Polypeptide toxins and many antibacterial peptides, such as colicins or hemolysins, and certain proteins involved in apoptosis, are sometimes considered a separate category. These proteins are water - soluble but can aggregate and associate irreversibly with the lipid bilayer and become reversibly or irreversibly membrane - associated. In genomes A large fraction of all proteins are thought to be membrane proteins. For instance, about 1000 of the ~ 4200 proteins of E. coli are thought to be membrane proteins. The membrane localization has been confirmed for more than 600 of them experimentally. The localization of proteins in membranes can be predicted very reliably using hydrophobicity analyses of protein sequences, i.e. the localization of hydrophobic amino acid sequences. See also Annular lipid shell Carrier protein Inner nuclear membrane proteins Ion channel Ion pump ( biology ) List of MeSH codes ( D12. 776 ) Receptor ( biochemistry ) Transmembrane proteins References Jump up ^ Johnson JE, Cornell RB ( 1999 ). `` Amphitropic proteins : regulation by reversible membrane interactions ( review ) ''. Mol. Membr. Biol. 16 ( 3 ) : 217 -- 235. doi : 10.1080 / 096876899294544. PMID 10503244. Jump up ^ Alenghat, Francis J. ; Golan, David E. ( 2013 ). `` Membrane Protein Dynamics and Functional Implications in Mammalian Cells ''. Current Topics in Membranes : 89 -- 120. doi : 10.1016 / b978 - 0 - 12 - 417027 - 8.00003 - 9. PMID 24210428. Jump up ^ Andreeva, A ( 2014 ). `` SCOP2 prototype : a new approach to protein structure mining ''. Nucleic Acids Res. 42 : D310 -- 4. doi : 10.1093 / nar / gkt1242. PMC 3964979. PMID 24293656. Jump up ^ Overington JP, Al - Lazikani B, Hopkins AL ( December 2006 ). `` How many drug targets are there ? ''. Nat Rev Drug Discov. 5 ( 12 ) : 993 -- 6. doi : 10.1038 / nrd2199. PMID 17139284. Jump up ^ Krogh, A. ; Larsson, B.R. ; Von Heijne, G. ; Sonnhammer, E.L.L. ( 2001 ). `` Predicting transmembrane protein topology with a hidden markov model : Application to complete genomes ''. Journal of Molecular Biology. 305 ( 3 ) : 567 -- 580. doi : 10.1006 / jmbi. 2000.4315. PMID 11152613. ^ Jump up to : Liszewski, Kathy ( 1 October 2015 ). `` Dissecting the Structure of Membrane Proteins ''. Genetic Engineering & Biotechnology News ( paper ). 35 ( 17 ) : 1, 14, 16 -- 17. ( subscription required ) Jump up ^ Almén, M. ; Nordström, K.J. ; Fredriksson, R. ; Schiöth, H.B. ( 2009 ). `` Mapping the human membrane proteome : A majority of the human membrane proteins can be classified according to function and evolutionary origin ''. BMC Biology. 7 : 50. doi : 10.1186 / 1741 - 7007 - 7 - 50. PMC 2739160. PMID 19678920. Jump up ^ Yibin, Lin ( 2013 ). `` The substitution of Arg149 with Cys fixes the melibiose transporter in an inward - open conformation ''. Biochimica et Biophysica Acta. 1828 ( 8 ) : 1690. doi : 10.1016 / j. bbamem. 2013.03. 003. Jump up ^ Von Heijne, G. ( 2006 ). `` Membrane - protein topology ''. Nature Reviews Molecular Cell Biology. 7 ( 12 ) : 909 -- 918. doi : 10.1038 / nrm2063. PMID 17139331. Jump up ^ Gerald Karp ( 2009 ). Cell and Molecular Biology : Concepts and Experiments. John Wiley and Sons. pp. 128 --. ISBN 978 - 0 - 470 - 48337 - 4. Retrieved 13 November 2010. Jump up ^ Selkrig J, Leyton DL, Webb CT, Lithgow T ( 2014 ). `` Assembly of β - barrel proteins into bacterial outer membranes ''. Biochimica et Biophysica Acta. 1843 ( 8 ) : 1542 -- 50. doi : 10.1016 / j. bbamcr. 2013.10. 009. PMID 24135059. ^ Jump up to : Daley, D.O. ; Rapp, M ; Granseth, E ; Melén, K ; Drew, D ; von Heijne, G ( 2005 ). `` Global topology analysis of the Escherichia coli inner membrane proteome ''. Science. 308 ( 5726 ) : 1321 -- 3. doi : 10.1126 / science. 1109730. PMID 15919996. External links Wikimedia Commons has media related to Membrane proteins. Look up membrane protein in Wiktionary, the free dictionary. Organizations Membrane Protein Structural Dynamics Consortium Membrane protein databases TCDB - Transporter Classification database, a comprehensive classification of transmembrane transporter proteins Orientations of Proteins in Membranes ( OPM ) database 3D structures of integral and peripheral membrane proteins arranged in the lipid bilayer Protein Data Bank of Transmembrane Proteins 3D models of all transmembrane proteins currently in PDB. Approximate positions of membrane boundary planes were calculated for each PDB entry. TransportDB Genomics - oriented database of transporters from TIGR Membrane PDB Database of 3D structures of integral membrane proteins and hydrophobic peptides with an emphasis on crystallization conditions Mpstruc database List of transmembrane proteins of known 3D structure. A curated list of transmembrane proteins from the Protein Data Bank MemProtMD is a database of around 3000 intrinsic membrane protein structures Proteins Processes Protein biosynthesis Post-translational modification Protein folding Protein targeting Proteome Protein methods Structures Protein structure Protein structural domains Proteasome Types List of types of proteins List of proteins Membrane protein Globular protein Globulin Albumin Fibrous protein Proteins : key methods of study Experimental Protein purification Green fluorescent protein Western blot Protein immunostaining Protein sequencing Gel electrophoresis / Protein electrophoresis Protein immunoprecipitation Peptide mass fingerprinting / Protein mass spectrometry Dual - polarization interferometry Microscale thermophoresis Chromatin immunoprecipitation Surface plasmon resonance Isothermal titration calorimetry X-ray crystallography Protein NMR Cryo - electron microscopy Freeze - fracture electron microscopy Bioinformatics Protein structure prediction Protein -- protein docking Protein structural alignment Protein ontology Protein -- protein interaction prediction Assay Enzyme assay Protein assay Secretion assay Display techniques Bacterial display mRNA display Phage display Ribosome display Yeast display Super-resolution microscopy Photoactivated localization microscopy Vertico SMI Structures of the cell membrane Membrane lipids Lipid bilayer Phospholipids Proteolipids Sphingolipids Sterols Membrane proteins Membrane glycoproteins Integral membrane proteins / transmembrane protein Peripheral membrane protein / Lipid - anchored protein Other Caveolae / Coated pits Cell junctions Glycocalyx Lipid raft / microdomains Membrane contact sites Membrane nanotubes Myelin sheath Nodes of Ranvier Nuclear envelope Phycobilisomes Porosomes Retrieved from `` https://en.wikipedia.org/w/index.php?title=Membrane_protein&oldid=811735033 '' Categories : Membrane biology Membrane proteins Hidden categories : Pages containing links to subscription - only content Talk Contents About Wikipedia Català Čeština Deutsch فارسی Français Galego 한국어 हिन्दी Italiano עברית ქართული Nederlands 日本 語 Polski Português Русский Simple English Slovenčina Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska Українська 中文 Edit links This page was last edited on 23 November 2017, at 16 : 13. 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Question: what are the different classes of membrane proteins
Answer: Integral polytopic proteins