Binding of cholera toxin to lipid bilayers of various compositions
Rissanen, Sami (2013)
Rissanen, Sami
2013
Teknis-luonnontieteellinen koulutusohjelma
Luonnontieteiden tiedekunta - Faculty of Natural Sciences
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Hyväksymispäivämäärä
2013-05-08
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201305231173
https://urn.fi/URN:NBN:fi:tty-201305231173
Tiivistelmä
During the last decades, several cholera pandemics have taken place. Still, cholera currently continues to be a major cause of morbidity and mortality making it a worldwide health problem. The causative agent of the disease is the bacterium V ibrio cholerae, and the toxin responsible for the symptoms is called cholera toxin (CT). CT is an AB5 hexameric enterotoxin consisting of two non-covalently bound parts: A and B subunits. The A subunit is responsible for the toxicity functions of CT while the B-pentamer is responsible for the binding of CT to the plasma membrane (PM) in the intestinal lumen of human body. The B-pentamer has been found to tether to GM1 gangliosides in the lipid rafts in PM. One way to investigate these kinds of events is to use fluorescence spectroscopy, where fluorescent markers, such as BODIPY dyes, are employed.
In this study, we investigate the effect of the composition of lipid bilayers on the binding of CT. For this purpose, we employ atomic-scale molecular dynamics simulations for a total of 2.8 microseconds. We consider two different lipid environments: a bilayer consisting of DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) in a liquid-disordered phase
(ld) and a bilayer consisting of SSM (N-stearoyl-d-erythro-sphingosylphosphorylcholine) and cholesterol in a liquid-ordered phase (lo). These membranes are enriched with both the native GM1 and the BODIPY-labeled GM1 resulting in four different lipid environments. In addition to four 500-nanosecond simulations of these membranes in the presence of the protein we also study the same four systems for 200 nanoseconds without the B-pentamer, this case serving as a reference.
The results show that cholera toxin binds rapidly to all simulated membranes. However, membrane properties important to the toxin binding were noticed to be dependent on the composition of the lipid bilayer. Electrostatic potential was observed to change significantly between the ld and lo phases. Also the geometry of the head group of GM1, essential for the receptor–ligand fitting, was noticed to change as the composition of the bilayer was altered. The results support the idea of CT binding to the GM1 ganglioside, and the membrane in the ld phase was noticed to be the most favorable for the toxin binding.
In this study, we investigate the effect of the composition of lipid bilayers on the binding of CT. For this purpose, we employ atomic-scale molecular dynamics simulations for a total of 2.8 microseconds. We consider two different lipid environments: a bilayer consisting of DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) in a liquid-disordered phase
(ld) and a bilayer consisting of SSM (N-stearoyl-d-erythro-sphingosylphosphorylcholine) and cholesterol in a liquid-ordered phase (lo). These membranes are enriched with both the native GM1 and the BODIPY-labeled GM1 resulting in four different lipid environments. In addition to four 500-nanosecond simulations of these membranes in the presence of the protein we also study the same four systems for 200 nanoseconds without the B-pentamer, this case serving as a reference.
The results show that cholera toxin binds rapidly to all simulated membranes. However, membrane properties important to the toxin binding were noticed to be dependent on the composition of the lipid bilayer. Electrostatic potential was observed to change significantly between the ld and lo phases. Also the geometry of the head group of GM1, essential for the receptor–ligand fitting, was noticed to change as the composition of the bilayer was altered. The results support the idea of CT binding to the GM1 ganglioside, and the membrane in the ld phase was noticed to be the most favorable for the toxin binding.