Abstract
The statistical properties of membrane protein random walks reveal information on the interactions between the proteins and their environments. These interactions can be included in an overdamped Langevin equation framework where they are injected in either or both the friction field and the potential field. Using a Bayesian inference scheme, both the friction and potential fields acting on the ε-toxin receptor in its lipid raft have been measured. Two types of events were used to probe these interactions. First, active events, the removal of cholesterol and sphingolipid molecules, were used to measure the time evolution of confining potentials and diffusion fields. Second, passive rare events, de-confinement of the receptors from one raft and transition to an adjacent one, were used to measure hopping energies. Lipid interactions with the ε-toxin receptor are found to be an essential source of confinement. ε-toxin receptor confinement is due to both the friction and potential field induced by cholesterol and sphingolipids. Finally, the statistics of hopping energies reveal sub-structures of potentials in the rafts, characterized by small hopping energies, and the difference of solubilization energy between the inner and outer raft area, characterized by higher hopping energies.
Highlights
The cell membrane is the interface where the communication between the environment and the cytosol takes place
We have shown that the toxin receptor in Madin-Darby canine kidney (MDCK) cells is recruited to confinement domains prior to toxin binding, at 37uC, the confinement domain area ranges from 0.01 to 0.8 mm2, the diffusivity in the domains is 0.13 mm2 s21, the motion of the receptor is not purely diffusive but is influenced by an interaction potential that, if approximated by a spring-like potential, is characterized by a mean spring constant of 0.40 pN.mm21(94 kBT.mm22) and that these domains are stable over periods longer than 10 minutes [22]
We have shown that cholesterol oxidase (CHOx) and sphingomyelinase (SMase) treatments drastically decrease the CPeT receptor confinement, whereas the cytoskeleton does not play a direct role in the confinement, which lead us to attribute the confinement domains to lipid raft platforms [22]
Summary
The cell membrane is the interface where the communication between the environment and the cytosol takes place. It is generally accepted that membranes of living cells harbor areas enriched in cholesterol and sphingolipids, namely lipid rafts, where many proteins important for signaling are concentrated [1,2,3,4,5,6,7,8,9,10,11,12,13]. These rafts are known to be more densely packed than their surroundings [14] and have been reported to be areas where the membrane thickness is larger [1,15]. Despite the enormous progress that has been accomplished, novel experimental data on the protein-lipid interactions is still highly desirable
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