Fluctuation Spectroscopy Methods for the Analysis of Membrane Processes

In the field of membrane lipid and protein dynamics, fluorescence recovery after photobleaching (FRAP) and single-particle tracking methods have provided ample evidence that lipids and proteins could have their motion restricted by interaction with other lipids and with the cell cytoskeleton.1–8 Fluorescence correlation spectroscopy (FCS) is a relatively new method in this field that has received particular attention recently because of the possible combination with super-resolution microscopy methods.9–14 In this context, the use of very small volumes of excitation or variable volumes of excitation15 was considered necessary to unravel to transport of molecules at the nanoscale. However, most of the FCS studies done so far are based on the original idea of measuring temporal correlation at a single point in the membrane. Measuring a single location in the membrane is restrictive since the temporal fluctuations at one point cannot reveal local microstructures or the anisotropic molecular transport in membranes. In this contribution, we discuss fluctuation methods based on spatial correlation that reveal the dynamics of membrane lipids and proteins at the nanoscale.16–27 We show here that spatial correlations intrinsically contain more information than the classical temporal correlation first introduced with the single-point FCS. Spatiotemporal correlation approaches have the potential to shift the paradigm in the use and kind of information that can be derived from fluctuation methods for membrane studies. Also, spatiotemporal correlation can complement single-particle tracking experiments with much higher sensitivity and faster time scale