User:Emma Wozniak/sandbox1
Bacterial Multidrug Efflux Transporter AcrB (1IWG)Bacterial Multidrug Efflux Transporter AcrB (1IWG)
Efflux pump systems have evolved in bacteria to act as a chemotherapeutic drug and antibiotic resistance mechanism within the bacterial cytoplasmic membrane. These pump systems typically consist of multiple proteins that are embedded in the membranes and periplasms of the bacterial cells, which all bond together to work to recognize and export foreign materials that have come in contact with the cell. They also play an essential role in biofilm formation of capable bacteria, and work to help provide protection and reduce the stress placed on the biofilm core.
1IWG is one component of a three-part AcrAB-TolC multidrug efflux pump system found in E. coli that works to maintain resistance against a range of antibiotics. This protein, which is also called the bacterial multidrug efflux transporter AcrB
|
protein, works together with the AcrA and TolC proteins to capture and export any antibacterial compounds and detergents that can compromise the cell. [1] 1IWG is the inner-membrane transporter protein in this system and has subunits that are present in both the cytoplasm and periplasm of the E. coli cell, which allows it to work in tandem with the TolC protein for drug export. It is believed that AcrB captures substrates mainly from the periplasm of the cell, but these substrates can still enter the inner cavity of this funnel-like protein from the cytoplasm.
Structural highlightsStructural highlights
This protein has a molecular mass of 114 kDa and is a one chain structure that consists of 1049 amino acids, and organizes itself as a homotrimer in shape similar to that of a jellyfish. Each protomer in this protein is composed of a 50 A thick transmembrane region, as well as a protruding headpiece that is 70 A in size. It is believed by some that this headpiece section opens up at the top like a funnel, which the TolC protein can directly dock itself onto. Direct disulfide cross-linking experiments have also shown the interactions between the AcrB and TolC trimers, suggesting that the headpiece-TolC belief might be true.
AcrB is shaped like a funnel with its ends found in both the cytoplasm and periplasm of the bacterial cell. The C terminus of AcrB is located in the cell’s cytoplasm, while the N terminus links with AcrA in the periplasm. The N- and C- terminal halves of AcrB are connected to each other with an alpha-helix that runs parallel to the cytoplasmic membrane of the bacteria. Subunits within the protein have three possible conformational states, which include the access (L), binding (T), and extrusion (O) states. However, binding of substrates and inhibitors will affect the conformational states of the protein. For example, binding of the MBX3132 inhibitor creates a TTT conformation state for the protein as a whole, which makes the inhibitor bind tightly to the protein and negatively affects substrate binding/protein function.[2]
It has first been found that a Asp-Lys-Asp triad was found to be essential in AcrB. Later, three individual subunits in the transmembrane region have been identified to play key roles in AcrB function. The has been found to be essential in the TM4 subunit in E. coli, and this residue works in conjunction with the and subunits to allow the protein to function. [3]
AcrAB-TolC Multidrug Efflux SystemAcrAB-TolC Multidrug Efflux System
The AcrAB-TolC multidrug efflux system
|
is a tripartite efflux pump system that belongs to the RND (Resistance Nodulation Division) transporter family. The RND transporter system is the most common efflux system present in Gram-negative bacteria, especially those of the Enterobacteriaceae family. The AcrAB-TolC multidrug efflux system found in E. coli allows the bacteria to recognize and export a wide range of diverse substrates, including antibiotics, bile salts in the mammalian gut, and biocides.
Efflux pumps in Gram-negative bacteria consist of one outer membrane protein, an inner membrane protein, and then a periplasmic protein that works to link the two trans-membrane components. In this particular pump system, TolC is the outer membrane protein that allows the E. coli to fully export the antibacterial agents, AcrB works as the inner membrane protein to collect substrates and initiate the exportation process, and AcrA is the periplasmic membrane-fusion protein. An additional transporter, AcrD, can also be found in this system and can interact directly with AcrA and TolC to export antibacterial agents under environmental stress and external stimuli. However, AcrB is the primary inner membrane protein in this efflux system. AcrB activity can be modified by the presence of a small additional protein called AcrZ, which affects substrate specificity of the AcrAB-TolC efflux pump through the allosteric changes on the AcrB protein. These allosteric changes occur in the transmembrane regions of the AcrB protein, which is what allows the protein to bind to and export a wide variety of substrates.
Biofilm FormationBiofilm Formation
Biofilm formation is a characteristic of a range of bacteria that are able to complete the process of quorum sensing (QS). QS is essentially an extracellular communication process between bacterial cells that allow them to control gene expression based on external stimuli. Bacteria will be able to respond appropriately to these external stimuli and create defense mechanisms, which include the formation of biofilms. Biofilms occur when a planktonic bacterial cell attaches itself to a surface, aggregates with other bacteria, and then produces an extracellular polymeric matrix. This extracellular matrix consists of a variety of substances, including polysaccharides, nucleic acids, proteins, and inorganic materials; these substances all work together to create a sticky layer of protection for the biofilm core. However, one problem that can occur within these biofilms is an accumulation of cell waste and substances that can become toxic to the bacteria within the biofilm core. Due to their ability to export toxic materials, efflux pumps have become an essential part in biofilms for protection of the cells found within the biofilm core. These efflux pumps can be found both in the biofilm core and extracellular matrix, allowing the waste to be pumped out of the biofilm entirely and maintain healthy conditions. [4] [5]
ReferencesReferences
- ↑ Nelson RE, Selitrennikoff CP, Siegel RW. Mutants of Neurospora deficient in nicotinamide adenine dinucleotide (phosphate) glycohydrolase. J Bacteriol. 1975 May;122(2):695-709. PMID:165174 doi:10.1128/jb.122.2.695-709.1975
- ↑ Yan H, Zhong G, Xu G, He W, Jing Z, Gao Z, Huang Y, Qi Y, Peng B, Wang H, Fu L, Song M, Chen P, Gao W, Ren B, Sun Y, Cai T, Feng X, Sui J, Li W. Sodium taurocholate cotransporting polypeptide is a functional receptor for human hepatitis B and D virus. Elife. 2012 Nov 13;1:e00049. doi: 10.7554/eLife.00049. PMID:23150796 doi:http://dx.doi.org/10.7554/eLife.00049
- ↑ Nelson RE, Selitrennikoff CP, Siegel RW. Mutants of Neurospora deficient in nicotinamide adenine dinucleotide (phosphate) glycohydrolase. J Bacteriol. 1975 May;122(2):695-709. PMID:165174 doi:10.1128/jb.122.2.695-709.1975
- ↑ Yamamoto K, Tamai R, Yamazaki M, Inaba T, Sowa Y, Kawagishi I. Substrate-dependent dynamics of the multidrug efflux transporter AcrB of Escherichia coli. Sci Rep. 2016 Feb 26;6:21909. doi: 10.1038/srep21909. PMID:26916090 doi:http://dx.doi.org/10.1038/srep21909
- ↑ Tanaka M, Hainu M, Yasunobu KT, Norton TR. Amino acid sequence of the Anthopleura xanthogrammica heart stimulant, anthopleurin A. Biochemistry. 1977 Jan 25;16(2):204-8. PMID:13806 doi:10.1021/bi00621a007
</StructureSection>