Anomalous Diffusion of Endoplasmic Reticulum Constituents

Abstract

Diffusion of macromolecules and higher-order structures in the crowded interior of cells frequently shows an anomalous behavior with the mean-square displacement (MSD) increasing nonlinearly in time, MSD∝tα. Here we have probed to which extent also larger organelle structures show such an anomalous diffusion and how non-equilibrium contributions affect their diffusional motion [Phys. Rev. E 98, 012406 (2018), in press: Biophys. J. 115 (2018)]. In particular, we have employed single-particle tracking to monitor the motion of tubular junctions in the endoplasmic reticulum (ER) network and of long-lived membrane domains on the ER, called ER exit sites (ERES). Our results show that both, ER junctions and ERES show a distinct anomalous diffusion with a significant anti-correlation of successive step increments that is reminiscent of fractional Brownian motion. Disrupting the microtubule cytoskeleton significantly altered the subdiffusive characteristics of both entities, highlighting that even anomalous diffusion is an actively driven process in living cells. While the diffusion behavior of ER junctions was seen to be directly dependent on the presence and activity of microtubules, ERES were only indirectly affected. ERES therefore can be seen as mobile membrane domains that perform a quasi-one-dimensional random walk on the shivering ER backbone.