Anomalous Diffusion of Cholera Toxin on Cell Membranes Investigated by Fluorescence Correlation Spectroscopy

Document Type


Degree Name

Master of Science (MS)



Date of Award

Spring 2019


Cholera is an acute disease produced by the action of cholera toxin (CT) causing severe diarrhea and the resultant an extreme loss of electrolytes. The toxin is secreted by the bacterium Vibrio cholerae, whose existence as the pathogen causing cholera was discovered in 1886, by Robert Koch, a renowned German epidemiologist, medical doctor and winner of the Nobel Prize for Physiology or Medicine [17]. In 1959, it was shown that CT produced an accumulation of rice-water liquid in tied ileal loops of rabbits [17]. A cholera victim will experience diarrhea, intensifying over time, followed by severe cramps, retching, thirst, including a change in skin color which was a combination of blue andgrey color, due to the substantial loss of fluid [12].CT consists of a holotoxin (AB5), whose B5 oligomer is not itself poisonous but stillcontributes to produce the symptoms of cholera. The A subunit is placed centrally while the fiveB subunits are positioned peripherally. The former is enzymatically toxic while the ladder bindsto the membrane. Cholera toxin’s action begins when its B subunits bind to five GMi receptorswhere endocytosis of CT takes place. Once CT reaches the endoplasmic reticulum (ER), the Asubunit disconnects from its B subunit to invade the target cell.Some studies suggested that the diffusion rate of CT is significantly slower than otherproteins of similar size and, thus, enable the entrance of toxin A subunit into intracellular space.Characterizing diffusion properties of cholera toxin B subunit, therefore, offers a means tounderstanding not only pathogenesis of cholera but also membrane environments that serve as amedium for cholera toxin B subunit diffusion.Biological diffusion is a fundamental process which occurs at almost all levels of thebiological system. Diffusion may occur as Brownian random movements of molecules, which isoften characterized by a linear relation between time and the mean square displacement, (x2) ~t. In particular, the 2-dimensional ideal (or normal) diffusion satisfies (x2) = 4Dt whereD(jimz/s) is a diffusion coefficient, a measure of how fast molecules diffuse over time. Due tothe complex cellular structure of cell membranes (included but not limited to transmembraneproteins, cytoskeletons, lift rafts, and carbohydrates), diffusion of proteins often shows deviationfrom a linear relation between time and the mean square displacement, which is referred to as ananomalous diffusion. Typically, anomalous diffusion can be characterized by the T-dependentdiffusion coefficient D(f). In this study, we developed a unique computational framework to measure D(t) of CTxB using fluorescent correlation spectroscopy to have a better understandingof the diffusion property of CTX as a potential drug target to prevent a cholera outbreak.


Minchul Kang

Subject Categories

Mathematics | Physical Sciences and Mathematics