The theoretical model further predicted that all proteins, whether with or without raft affinity, diffuse significantly slower when rafts are present. In this context, it is interesting to note that protein diffusion in cell membranes is several times slower than in artificial bilayer constructs composed of just a few lipid species. In SPT 31 , individual molecules are labelled, successively imaged, tracked, and the trajectories analysed 32 , To map the entire cell surface, localisation and connecting molecules into trajectories have to be achieved for a high density of molecules.
To date, no standardised tracking algorithm exists that can simultaneously account for high particle density, heterogeneous particle motion, particle interactions merging and splitting , or temporary disappearance e. Progress has been made 33 , 36 , but the interpretation of trajectories will still rely on simulations 37 until robust algorithms are shared between researchers. The advantage of SPT is that it not only allows molecular diffusion coefficients to be calculated but different modes of diffusion of the same molecule can be identified.
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Different modes of motion—immobile, free, directed, and confined—were all observed for the same receptor. Transient confinement zones TCZs , detected by SPT define areas where an observed molecule stays much longer than expected from the average diffusion coefficient and is thought to bear some resemblance to lipid rafts. Yet, whether the viscosity differential inside and outside the raft is a sufficient diffusion barrier is questionable 12 , particularly since TCZs appear to be temperature independent These three constructs had similar diffusion coefficients assuming Brownian motion , but two populations of diffusing molecules were observed The major population displayed similar diffusion times to small molecule membrane dyes 0.
In this instance, cholesterol depletion did not affect confinement. Similar to the size of the observation area in FRAP, the sampling frequency in SPT experiments can also influence values of the diffusion coefficients and data interpretation.
Proteins and lipids are slowed down as they hop from one compartment to the next, but within each compartment diffusion is not significantly lower than that observed for free diffusion. It should be remembered that to distinguish between Brownian motion and hop diffusion requires a time resolution of tens of microseconds, without which hop diffusion is simply interpreted as free diffusion with a slower average diffusion coefficient. A key finding of raft characteristics by the theoretical calculation by Nicolau et alis that global diffusion retardation by high viscosity islands increases the rate of protein—protein interactions Raft domains of that nature also modestly increase the collision rate of proteins even if they have no affinity for the raft domain.
The authors point out that this may have important biological consequences for different types of membrane domains, e. An alternative single molecule technique is FCS Figure 4 , first demonstrated by Elson in as a method for analysing molecular kinetics The method uses low laser powers and low probe concentrations making it applicable to live cell measurements with minimised perturbations to the membrane.
Recently, several related techniques have been developed and continue to be developed. These new methods have the ability to map the spatial distribution of diffusion coefficients at selected membrane regions, as demonstrated for the mobility of paxillin at focal adhesions A As single molecules diffuse into the stationary spot illuminated by a confocal microscope, bursts of fluorescence are detected. The advantage of FCS is that it allows one not only to extract diffusion coefficients and identify anomalous diffusion but that it also measures the number of molecules in the observed spot.
For example, fluorescence fluctuation analysis has been used to determine the degree of clustering of the EGF receptor 22 and the stoichiometry of protein complexes Microdomains and cytoskeletal confinement impart different deviations on the relationship between the diameter of the observation spot and transit time that a molecule takes to diffuse through that spot This approach has since been used to demonstrate that the protein kinase Akt dynamically partitions into microdomains in cells via its pleckstrin homology PH domain Further technical improvements for FCS measurements specific to membranes are summarised in a recent review This allows the correlation of dependencies between various fluorescence parameters.
For example, FCS autocorrelation curves can be constructed for fluorescent events of a certain fluorescence lifetime. This can not only help to remove background and autofluorescence and thus makes diffusion measurements more accurate, but also enables a single experiment to give information about the kinetics of, for example, monomers and dimers, local concentrations of proteins and their different states, their relative stoichiometry, and affinities.
Structure and Interactions of Lipid Bilayers: Role of Fluctuations | SpringerLink
By combining this information with mathematical models, we are in a position to take a big step forward towards a detailed understanding how proteins function in the cell membrane. The recent technical advances in fluorescence microscopy hardware and analysis have enabled researchers to quantify the dynamics of membrane proteins in live cells. FRAP, FCS, and SPT go beyond traditional fluorescence imaging and have converted microscopes into molecular measurement tools that can operate on many spatial and temporal length scales. Synergistically, these approaches have helped develop a picture where the diffusion characteristics of membrane proteins show complex and subtle behaviour.
The interdependence of membrane organisation and protein diffusion is, however, still poorly understood. Furthermore, current models do not adequately take into account active transport systems such as directed flow, membrane budding, recycling, and so forth. So far, we have a limited understanding of how local variations in membrane fluidity affect protein concentration, oligomerisation, and diffusion. Volume 10 , Issue 8.
Tight cohesion between glycolipid membranes results from balanced water–headgroup interactions
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Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Abstract The mobility of membrane proteins is a critical determinant of their interaction capabilities and protein functions. Figure 1 Open in figure viewer PowerPoint. Figure 2 Open in figure viewer PowerPoint. Figure 3 Open in figure viewer PowerPoint.
Figure 4 Open in figure viewer PowerPoint. Conclusions and Perspectives The recent technical advances in fluorescence microscopy hardware and analysis have enabled researchers to quantify the dynamics of membrane proteins in live cells. The fluid mosaic model of the structure of cell membranes. Science ; : — Cell membranes and the cytoskeleton P.
Electroporation and electrofusion of membranes D. Cation-induced vesicle fusion modulated by polymers and proteins K. Author index. Subject index. The first volume of the Handbook deals with the amazing world of biomembranes and lipid bilayers. Part A describes all aspects related to the morphology of these membranes, beginning with the complex architecture of biomembranes, continues with a description of the bizarre morphology of lipid bilayers and concludes with technological applications of these membranes. The first two chapters deal with biomembranes, providing an introduction to the membranes of eucaryotes and a description of the evolution of membranes.
The following chapters are concerned with different aspects of lipids including the physical properties of model membranes composed of lipid-protein mixtures, lateral phase separation of lipids and proteins and measurement of lipid-protein bilayer diffusion. Other chapters deal with the flexibility of fluid bilayers, the closure of bilayers into vesicles which attain a large variety of different shapes, and applications of lipid vesicles and liposomes.
Part B covers membrane adhesion, membrane fusion and the interaction of biomembranes with polymer networks such as the cytoskeleton. The first two chapters of this part discuss the generic interactions of membranes from the conceptual point of view. The following two chapters summarize the experimental work on two different bilayer systems. The next chapter deals with the process of contact formation, focal bounding and macroscopic contacts between cells.
The cytoskeleton within eucaryotic cells consists of a network of relatively stiff filaments of which three different types of filaments have been identified. As explained in the next chapter much has been recently learned about the interaction of these filaments with the cell membrane.
The final two chapters deal with membrane fusion. Many of the topics covered here, such as the role of surface tension, the physical basis of the interactions between surfaces and the electrostatic properties of membranes, have not been extensively reviewed elsewhere. For those with a more general interest in the properties of membranes, the volume will provide excellent coverage of a less familiar literature.
Structure and Dynamics of Membranes (eBook)
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