  Diffusion of tracer particles in the cytoplasm of mammalian cells is often
anomalous with a marked heterogeneity even within individual particle
trajectories. Despite considerable efforts, the mechanisms behind these
observations have remained largely elusive. To tackle this problem, we
performed extensive single-particle tracking experiments on quantum dots in the
cytoplasm of living mammalian cells at varying conditions. Analyses of the
trajectories reveal a strong, microtubule-dependent subdiffusion with
antipersistent increments and a substantial heterogeneity. Furthermore,
particles stochastically switch between different mobility states, most likely
due to transient associations with the cytoskeleton-shaken endoplasmic
reticulum network. Comparison to simulations highlight that all experimental
observations can be fully described by an intermittent fractional Brownian
motion, alternating between two states of different mobility.
