The translational mobility of substances within the cytoplasmic matrix.

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Abstract
The translational mobility of fluorescent-labeled molecules injected into the cytoplasm of [human] living cells can be measured by the fluorescence recovery after photobleaching (FRAP) technique. In the fibroblast cytoplasm, the diffusion coefficients, D, of test macromolecules ranging in MW from 12,000-440,000 are .apprx. 10-8 cm2/s and exhibit almost no dependence on MW. FRAP experiments also showed that macromolecular diffusion within Sepharose beads having an effective pore size smaller than the microtrabecular lattice is only slightly retarded compared to buffer values, in contrast to the marked retardation measured in the cytoplasm. Diffusion in the cytomatrix is dominated not by steric effects but rather by binding of the diffusing species to elements of the cytomatrix. These diffusion rates were difficult to modulate; cytochalasin, colchicine (except at 5.degree. C) and taxol treatments had little effect. The diffusion rates were not dependent on cellular energy metabolism. Hypotonic treatment increased the D for bovine serum albumin by nearly 2-fold, whereas hypetonic treatment halved D. Withdrawing the free water from the cell by using 44% polyethylene glycol treatment stopped the translational mobility of the test molecules. A survey of the recent literature is presented, which shows that major differences in the cytomatrix of different cell types exist with respect to the translational diffusion of injected probes. The spectrum of cytoplasmic transitional mobilities ranging from small molecules to organelles is discussed.