Diffusion of exit sites on the endoplasmic reticulum – a random walk on a shivering backbone

Abstract

Major parts of the endoplasmic reticulum (ER) in eukaryotic cells are organized as a dynamic network of membrane tubules connected by three-way junctions. On this network, self-assembled membrane domains, called ER exit sites (ERES), provide platforms at which nascent cargo proteins are packaged into vesicular carriers for subsequent transport along the secretory pathway. While ERES appear stationary and spatially confined on long time scales, we show here via singleparticle tracking that they exhibit a microtubule-dependent anomalous diffusion behavior on short and intermediate time scales. By quantifying key parameters of their random walk, we show that the subdiffusive motion of ERES is distinct from that of ER junctions, i.e. ERES are not tied to junctions but rather are mobile on ER tubules. We complement and corroborate our experimental findings with model simulations that also indicate that ERES are not actively moved by microtubules. Altogether, our study shows that ERES perform a random walk on the shivering ER backbone, indirectly powered by microtubular activity. Similar phenomena can be expected for other domains on subcellular structures, setting a caveat for the interpretation of domain tracking data.

Diffusive search and trajectories on tubular networks: a propagator approach

“…Proteins destined for secretion are co-translationally inserted into the ER membrane or lumen, undergo folding and quality control [45,46], and must leave the ER through punctate ER exit sites (ERES). These ERES are largely immobile sites scattered throughout the network [47] (Fig. 2b) and proteins are assumed to diffuse to one of these sites for capture and packaging into vesicles that enable them to leave the ER and proceed to further steps of secretory processing [48,49].…”

Unscrambling exit site patterns on the endoplasmic reticulum as a quenched demixing process

“…Moreover, ERESs can be monitored over extended timescales, during which they maintain their average size with a diameter of some 100 nm (8). Although ERES positions appear stationary on large timescales, they show a distinct subdiffusional motion on short and intermediate timescales (34). In fact, their motion is distinct from that of ER junctions (20), i.e., ERESs appear to perform a one-dimensional random walk on shivering ER tubules that are shaken by the active cytoskeleton (19,20).…”

Dynamic Capture and Release of Endoplasmic Reticulum Exit Sites by Golgi Stacks in Arabidopsis

“…On the other hand, ERES structures similar to beaded ring-shaped ERESs were also detected in mammalian cells (McCaughey et al, 2016) and in D. melanogaster (Liu et al, 2017). Additionally, a simulation of the dynamics of ERESs shows that ERESs randomly move around the ER tubules and are eventually confined by cup-shaped domains of the ER in mammalian cells (Stadler et al, 2018), as in A. thaliana (Figure 6). Thus, our model of capture and release of punctate ERESs by Golgi stacks might be applicable to the ER-to-Golgi transport in mammals and in Drosophila.…”