Diffusion in crowded fluids, e.g. in the cytoplasm of living cells, has frequently been reported to show anomalous characteristics (socalled 'subdiffusion'). Several random walk models have been proposed to explain these observations, yet so far an experimentally supported decision in favor of one of these models has been lacking. Here, we show that experimentally obtained trajectories in a prototypical crowded fluid show an asphericity that is most consistent with the predictions of fractional Brownian motion, i.e. an anticorrelated, anti-persistent generalization of normal Brownian motion that is related to the fluid's viscoelasticity.

A key event in many cellular signaling cascades is the multiple phosphorylation of proteins by specialized kinases. A prototypical example is the mitogen-activated protein kinase (MAPK) that alters the cell's gene transcription after having been phosphorylated twice by the same kinase. Here, we show that anomalous diffusion, induced, for example, by cytoplasmic crowding, can significantly improve the activation of MAPK. Our results on anomalous diffusion with the characteristics of fractional Brownian motion and obstructed diffusion compare favorably to very recent biochemical data on MAPK activation at varying degrees of cytoplasmic crowding. Our results predict any Michaelis-Menten scheme in which a substrate is modified by the same enzyme several times to show an increased performance due to anomalous diffusion when dissociation rates of the intermediate enzyme-substrate complexes are high while the irreversible catalytic step is slow. Thus, crowding-induced anomalous diffusion can strongly alter the behavior of many cellular signaling pathways.

Anomalous diffusion in crowded fluids, e.g. in the cytoplasm of living cells, is a frequent phenomenon. Despite manifold observations of anomalous diffusion with several experimental techniques, a thorough understanding of the underlying microscopic causes is still lacking. Here, we have quantitatively compared two popular techniques with which anomalous diffusion is typically assessed. Using extensive computer simulations of two prototypical random walks with stationary increments, i.e. fractional brownian motion and obstructed diffusion, we find that single particle tracking (SPT) yields results for the diffusion anomaly that are equivalent to those obtained by fluorescence correlation spectroscopy (FCS). We also show that positional uncertainties, inherent to SPT experiments, lead to a systematic underestimation of the diffusion anomaly, regardless of the underlying random walk and measurement technique. This effect becomes particularly relevant when the position uncertainty is larger than the average positional displacement between two successive frames.