Dynamics of and on the endoplasmic reticulum*

The interior of eukaryotic cells is highly compartmentalized
by a variety of organelle structures. A particularly prominent
organelle is the endoplasmic reticulum (ER) that feeds virtually
all secretory processes of a cell (amongst other duties). Large
portions of the ER are organized as a vast dynamic network of
membrane tubules that are connected by three-way junctions.
Dispersed on this network are numerous membrane domains, so-called
ER exit sites (ERES), at which nascent proteins leave the ER via
transport vesicles to get transported along the cell's secretory
pathway.
Using time-resolved imaging, we have analyzed the dynamics of ER
junctions and of ERES domains as well as the self-organization of
ERES on the ER network. As a result, we observed that both, ER
junctions and ERES, show a marked subdiffusion with signatures of
an anti-persistent fractional Brownian motion and a strong dependence
on the cytoskeleton's integrity: ER junctions move like monomer units
of (semi)flexible polymers with the overall dynamics of the ER network
being governed by fractons due to the network's self-similar geometry.
Moreover, active cytoskeleton-associated processes endow the subdiffusion
of ER junctions with a distinct nonequilibrium character. In contrast,
ERES rather are mobile domains that perform a quasi-one-dimensional
random walk on the shivering ER backbone. Moreover, ERES show a
self-organization that is reminiscent of two-dimensional droplet
formation where coarsening is hindered by geometric and topological
constraints imposed by ER junctions. In line with this view, a
substantial coarsening of ERES to fewer but larger domains is
observed upon lifting the hindrance via an RNAi-induced reduction
of ER junctions.