Biomedical Engineering Reference
In-Depth Information
sucrose synthase is abundantly located just below the plasma membrane where the CSC
is located (41, 42). The relationship of sucrose synthase to CesA may have become more
specialized in wood and cotton fibers, which synthesize massive amounts of cellulose,
because Arabidopsis growth was unaltered (under normal conditions) when six sucrose
synthase isoforms were down-regulated singly or in pairs of related genes in Arabidopsis
(43). However, a mechanism for preferential transfer of UDP-glucose from sucrose
synthase to at least some isoforms of CesA is not known. High concentrations of
sucrose promote association of sucrose synthase with membranes (44), which establishes
a mechanistic basis for the idea that high rate cellulose synthesis supported by sucrose
synthase would occur more frequently under ideal growth conditions when abundant
fixed carbon is available (45).
2.4.4
Polymerize Glucose with ß-1,4-Linkage
Although CS proteins in different lineages have low overall amino acid sequence homol-
ogy, they share similar structure and common domains associated with their catalytic
function as glycosyltransferases in the GT2 family that are capable of adding glucose
derived from UDP-glucose to a growing polymer. CesA is thought to polymerize glucose
in a processive manner, meaning that it remains bound to the product during successive
polymerization steps. To account for multiple CesA isoforms in most land plants and to
rationalize some data, it has been proposed that certain CesAs may generate a short-chain,
possibly lipid-linked, primer, with other CesAs perhaps facilitating the polymerization
of high molecular weight cellulose (39, 46). Data consistent with a lipid-linked glucan
primer as part of cellulose synthesis are still reported (see discussion in (32)), but the
actual biochemical product
in vivo
of any of the plant CesA isoforms is unproven.
The ß-1,4 linkage distinguishes cellulose from other glucose homopolymers, includ-
ing starch (with
α
-1,4 glucan backbone) and callose (with ß-1,3 glucan backbone) and
generates the flat-chain conformation that facilitates crystallization of extended parallel
chains into cellulose I. This linkage implies, however, that alternate glucose residues are
flipped 180
◦
relative to each other so that the actual repeating unit is cellobiose. This
poses special challenges for polymerase activity. One possible explanation is that for
each forming polymer chain, paired active sites from two CS isoforms exist in the CSC
to hold glucose in opposite orientation (8, 31, 47, 48). Data consistent with this idea
exist for the analogous systems of chitin and hyaluronan synthase (reviewed in (49)).
2.4.5
Operate so that Fibrils Emerge Outside the Plasma Membrane
Since substrate UDP-glucose is acquired in the cytoplasm, it is possible that the aggre-
gated TMH of the CesA protein(s) create a pore in the membrane to allow elongating
glucan chains to exit on the cell wall side. Data to demonstrate the transport mechanism
for polymerized ß-1,4 glucan are lacking.
2.4.6
Control Cellulose Chain Length
Cellulose chain length is determined by the degree of polymerization (DP) and is vari-
able during development as shown by analysis of single-celled cotton fiber. During
primary wall deposition, cotton fiber cellulose has a wider length distribution, averaging
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