Fluorescence Correlation Microscopy (FCM): Fluorescence Correlation Spectroscopy (FCS) in Cell Biology. Fluorescence Correlation Spectroscopy. Berlin: Springer Berlin Heidelberg; 2001. pp. 132-61. [DOI: 10.1007/978-3-642-59542-4_7]

Distribution of resting and ligand-bound ErbB1 and ErbB2 receptor tyrosine kinases in living cells using number and brightness analysis

Ligand-driven dimerizations of ErbB receptor subunits fulfill a fundamental role in their activation. We have used the number and brightness analysis technique to investigate the existence of preformed ligand-independent dimers and clusters and to characterize the initial steps in the activation of ErbB1 and ErbB2. In cells expressing 50,000-200,000 receptors, ErbB1 was monomeric in the absence of ligand stimulation, whereas in CHO cells with receptor levels >500,000 as much as 30% of ErbB1 was present as preformed dimers. EGF induced the formation of ErbB1 dimers as well as larger clusters (up to pentamers) that colocalized with clathrin-coated pits. The distribution of unstimulated ErbB2 in cells expressing 3·10(5)-10(6) receptors was fundamentally different, in that this receptor was present in preformed homoassociated aggregates containing 5-10 molecules. These constitutive ErbB2 homoclusters colocalized with caveolae, increased in size at subphysiological temperatures, but decreased in size upon EGF stimulation. We conclude that these ErbB2 clusters are promoted primarily by membrane-mediated interactions and are dispersed upon ligand stimulation.

Translational and rotational motions of proteins in a protein crowded environment

Fluorescence correlation spectroscopy (FCS) was used to measure the translational diffusion of labeled apomyoglobin (tracer) in concentrated solutions of ribonuclease A and human serum albumin (crowders), as a quantitative model system of protein diffusive motions in crowded physiological environments. The ratio of the diffusion coefficient of the tracer protein in the protein crowded solutions and its diffusion coefficient in aqueous solution has been interpreted in terms of local apparent viscosities, a molecular parameter characteristic for each tracer-crowder system. In all protein solutions studied in this work, local translational viscosity values were larger than the solution bulk viscosity, and larger than rotational viscosities estimated for apomyoglobin in the same crowding solutions. Here we propose a method to estimate local apparent viscosities for the tracer translational and rotational diffusion directly from the bulk viscosity of the concentrated protein solutions. As a result of this study, the identification of protein species and the study of hydrodynamic changes and interactions in model crowded protein solutions by means of FCS and time-resolved fluorescence depolarization techniques may be expected to be greatly simplified.

Fluorescence correlation spectroscopy measures molecular transport in cells

Fluorescence correlation spectroscopy (FCS) can measure dynamics of fluorescent molecules in cells. FCS measures the fluctuations in the number of fluorescent molecules in a small volume illuminated by a thin beam of excitation light. These fluctuations are processed statistically to yield an autocorrelation function from which rates of diffusion, convection, chemical reaction, and other processes can be extracted. The advantages of this approach include the ability to measure the mobility of a very small number of molecules, even down to the single molecule level, over a wide range of rates in very small regions of a cell. In addition to rates of diffusion and convection, FCS also provides unique information about the local concentration, states of aggregation and molecular interaction using fluctuation amplitude and cross-correlation methods. Recent advances in technology have rendered these once difficult measurements accessible to routine use in cell biology and biochemistry. This review provides a summary of the FCS method and describes current areas in which the FCS approach is being extended beyond its original scope.