Biogenesis of Cellulose Nanofibrils by a Biological Nanomachine - The Nanoscience and Technology of Renewable Biomaterials - page 48

emerge outside the plasma membrane; control cellulose chain length and fibril size;
possibly control cellulose crystallization; and move in the plasma membrane to spin out
cellulose fibrils. We will consider each of these functions in order to illustrate both the
remarkable capabilities of this cellular fibril-spinning nanomachine and the extensive
lack of mechanistic information that currently exists. Readers wishing to consider any
of these functions in detail are encouraged to read the cited references as well as recent
reviews (12-16).

2.4.1 Assemble with Genetically Determined Morphology

As reviewed by Tsekos (1999), there are a variety of CSC morphologies in nature, and
it is evident that these are genetically determined. The rosette CSC is known so far
to exist only in the land plants and the charophycean algae. Its existence is correlated
with an increase in the number of CesA genes in these lineages, although there may not
be a causal relationship (see the section on phylogenetic analysis below). In flowering
plants (angiosperms), there is experimental evidence that different types of CesA pro-
teins (isoforms) are required for cellulose synthesis and CSC assembly (see review (12)).
Genetic analysis of mutants with cellulose deficient phenotypes in Arabidopsis showed
that AtCesA1, 3, and 6 (or 6-like proteins) are all required for primary wall cellulose
synthesis (17-21), whereas AtCesA4, 7, and 8 are all required for secondary wall cellu-
lose synthesis in xylem and fibers (22-24). Various phylogenetic analyses (25-28) group
the CesA genes from angiosperms into six clades (or sequence groups with a common
evolutionary origin) (Figure 2.3). Each of these six clades includes one of the three
Arabidopsis CesA genes required for primary wall synthesis/CSC assembly ( AtCesA1,
3, or 6-like ; defining clades P1, P2, or P3, respectively) or one of the three CesA genes
required for secondary wall synthesis/CSC assembly ( AtCesA4, 7, or 8 ; defining clades
S1, S2, of S3, respectively). The six clades also include orthologs from other angiosperm
taxa, and mutant analysis in other angiosperm species is generally consistent with the
results from Arabidopsis. (Orthologs are genes in two or more species that evolved from
a common ancestor and typically retain the same function.)
Doblin et al . (2002) (29) proposed a modification of a previous model (30) that
explains the geometry of rosette CSCs as a function of the inter- and intra-particle inter-
action between three distinct CesA subunits that associate with each other through distinct
binding sites. For both the primary and secondary wall cases, there is accumulating evi-
dence that each member of a CesA triad plays a distinct role in the assembly of rosettes
(17-24, 30-32). First, inhibition of cellulose crystallization in the temperature-sensitive
Arabidopsis cesA1 ( rsw1 ) mutant, which has a single amino acid substitution in the cyto-
plasmic loop, was accompanied by depletion of rosette CSCs in the plasma membrane
(33). When grown at the restrictive temperature, AtCesA1, 3, and 6 behaved indepen-
dently during immunoprecipitation experiments rather than in a coordinated

840 kDa
complex as was found in plants grown at the permissive temperature (32). Second,
specific association between CesA subunits from both Arabidopsis and cotton has been
demonstrated in vitro (17, 22-24, 32, 34). The ZnBD is able to mediate the in vitro
association of cotton CesA proteins either as homo- or heterodimers, directly implicating
this feature in rosette assembly. For cotton GhCesA1, dimerization is regulated by the
redox state of the ZnBD (34).

∼

Third, interaction in vivo between all possible pairs of