Biogenesis of Cellulose Nanofibrils by a Biological Nanomachine - The Nanoscience and Technology of Renewable Biomaterials - page 50

2.4.2 Stabilize in Operational Form in the Plasma Membrane

The assembly of the rosette CSCs occurs in the plant endomembrane system. The
organized rosette was seen by FF-TEM in cells synthesizing secondary walls within distal
areas of Golgi cisternae, as well as in small vesicles free in the cytoplasm and apparently
fusing with the plasma membrane (1). When one member of the triad of secondary
cell wall CesA proteins is absent, the remaining two CesA proteins accumulate in the
Golgi (24). Fluorescently labeled AtCesA6 has also been observed to traffic through the
Golgi compartment during primary wall synthesis (36). Presently we do not know how
CSC activity is prevented in vascular plants prior to its incorporation into the plasma
membrane.
Specific factors also maintain the stability of preassembled rosettes after their insertion
into the plasma membrane, as indicated by FF-TEM images of cultured plant cells that
were synthesizing secondary walls when they were treated with a cellulose synthesis
inhibitor. In this case, assembled rosette CSCs were observed in the membrane, but they
appeared to expand in diameter through separation of the six subunits, then disappear
quickly as organized entities (35). In the absence of immunolabeling, it was not possible
to discern the fate of individual subunits after the CSC dispersed. Similarly, another
cellulose synthesis inhibitor caused detectable fluorescently labeled AtCesA6 to clear
from the membranes of cells synthesizing primary walls (36).
Older cell biological evidence and recent biochemical experiments with GhCesA1
from cotton fiber show that the half-life of a CesA in the plasma membrane is < 30 min,
which is unusually short for a membrane protein (37). The functional significance of this
short half-life is not yet known. Although CesA proteins could be recycled between the
plasma membrane and the endomembrane system, recent data show that they can also
be proteolytically degraded. Targeting for degradation can be enhanced for AtCesA7 by
phosphorylation within the catalytic domain (38), and monomeric ZnBDs (and, poten-
tially, intact CesA proteins) are also more rapidly degraded (37). It is not known how
rapid turn-over in CesA proteins regulates the outcomes of CSC function in vivo .

2.4.3 Acquire UDP-Glucose Substrate

As in bacteria, UDP-glucose is the immediate substrate for plant cellulose synthesis.
Common catalytic function within the GT2 family is reflected by the conservation of
three aspartate (D) residues and a QxxRW motif embedded within the conserved U1,
U2, U3 and U4 regions (7); (Figure 2.2). (QxxRW signifies five amino acids: glutamine,
two variable residues, arginine, and tryptophan.) The heterologously expressed cotton
fiber CesA protein (GhCesA1) bound UDP-glucose (9), and, using UDP-glucose sub-
strate, glucan polymerization was initiated from a putative primer (sitosterol-glucoside)
by membranes from yeast expressing the GhCesA1 gene (39). Cleavage of the UDP
moiety releases energy that can be used for addition of a liberated glucose residue to a
growing ß-1,4-linked glucan chain.
There is evidence that some cells with secondary walls, such as cotton fiber and xylem
sclerenchyma, preferentially accept UDP-glucose generated by sucrose synthase even
though UDP-glucose is usually found within a free pool in the cytoplasm (40). Despite
its name, sucrose synthase most commonly degrades sucrose to release UDP-glucose
and fructose in heterotrophic cells. In cells synthesizing cellulose-rich secondary walls,