Endosome-ER Interactions Define a Cellular Energy Landscape to Guide Cargo Transport

Molecular motor-driven cargo must navigate a complex intracellular environment, which is often crowded, heterogeneous and fluctuating, to fulfill their diverse functions in a cell. To meet these challenges, most cargo display qualitatively similar transport behavior, that is, random switching between states of diffusive "jiggling" movement (off-state) and states of directed "runs" (on-state). The physical picture of this 2-state motion and their regulation in a cell are not well understood. Here, by using single-particle tracking and motion-states dissection, we present a statistical analysis of the 2-state motion of epidermal growth factor receptor (EGFR)-endosomes in living cells. From a thorough analysis of a large volume of EGFR-endosome trajectories, we reveal that their lifetime in both states feature an exponential distribution with its probability density function (PDF)-amplitude varied by more than three decades. We show that their characteristic time, on-state probability, velocity and off-state diffusion coefficient are spatially regulated, and are probably contributed by the endoplasmic reticulum (ER) network via its spatially varying membrane densities and interactions with the cargo. We further propose a 2-state transport model to describe the complex, spatially varying transport dynamics of EGFR-endosomes in a cell. Our findings suggest that the ER network may play an essential role in constructing a cellular-level free-energy landscape {Delta}G(r) to spatially guide cargo transport.