Myosin XI functions in endoplasmic reticulum
(ER) dynamics
The ER is a membrane-enclosed organelle in eukaryotes.
The ER develops an elaborate polygonal network composed
of tubules and cisternae in the cell periphery,
which is frequently remodeled through tubule growing/
shrinking, sliding junctions, and cisternal expansion/
contraction [6–9]. In addition, plant ER develops thick
strand-like structures (Figure 1a). Previous studies
reported rapid ER streaming in the strands[10–12], which
is characteristic of plant ER dynamics. Velocity maps
depicted by KbiFlow software [13] reveal ER motility
with velocities ranging from 0.7 mm/s (in most peripheral
region of cell) to 4.2 mm/s (in the cell interior)
within a single cell (Figure 1b). The ER streams 6-fold
faster in the cell interior than in the peripheral region.
ER motility in plant cells is primarily driven by the actin–
myosin XI cytoskeleton and secondarily by the microtubule
cytoskeleton [14]. The myosin XI motors responsible
for ER motility have been identified in recent years.
A myosin XI member in tobacco BY-2 cultured cells
(175-kDa myosin XI) is associated with the ER [15]. In
an in vitro ER tubule formation assay with the cytosol-andmicrosome
fraction of BY-2 cells and exogenously added
ATP, GTP, and F-actin, depletion of 175-kDa myosin XI
significantly inhibited tubule formation from small ER
vesicles [16]. These results clearly demonstrate that
the ER-associated 175-kDa myosin XI provides a motive
force to elongate ER tubules in tobacco cells.
Reverse genetic analysis of A. thaliana identified three
myosin XI members responsible for the ER streaming;
myosin XIk is a primary contributor, whereas the contributions
of myosins XI1 and XI2 are more limited in
cotyledonary petioles [13]. The involvement of myosin
XI in ER network dynamics has been analyzed with
persistency maps and by expressing a motor-lesstruncated
tail of myosin XI, which is thought to disturb endogenous
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