In Vitro Reconstitution of the Endoplasmic Reticulum

Anomalous dynamics of the endoplasmic reticulum network

Large portions of the endoplasmic reticulum (ER) in eukaryotic cells are organized as dynamic networks whose segments are connected by three-way junctions. Here we show that ER junctions move subdiffusively with signatures of fractional Brownian motion and a strong dependence on the cytoskeleton's integrity: The time-averaged mean square displacement scales as 〈r^{2}(τ)〉_{t}∼τ^{α} with α≈0.5 in untreated cells and α≈0.3 when disrupting microtubules, with successive steps being anticorrelated in both cases. We explain our observations by considering ER junctions to move like monomers in (semi)flexible polymer segments immersed in a viscoelastic environment. We also report that ER networks have a nontrivial fractal dimension d_{f}≈1.6 on mesoscopic scales and we provide evidence that the organelle's dynamics is governed by fractons.

Diffusion of Exit Sites on the Endoplasmic Reticulum: A Random Walk on a Shivering Backbone

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. Although ERES appear stationary and spatially confined on long timescales, we show here via single-particle tracking that they exhibit a microtubule-dependent and heterogeneous anomalous diffusion behavior on short and intermediate timescales. 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.