Biomedical Engineering Reference
In-Depth Information
Figure 2.2 Protein structure diagrams showing domains within representative CS pro-
teins from Gossypium hirsutum (U58283.1, vascular plant type), Anabaena variabilis
(YP 322086.1,cyanobacterialtype),Dictyostelium discoideum (AF163835.1,protistantype),
and Gluconoacetobacter xylinus (CAA38487.1, eubacterial type). Thediagramsarealigned
at theU3 region. Domains shown by color blocks are: zinc-binding domain (Zn, yellow);
transmembrane domains (TMD, black), each of which contains multiple TMH; CS protein
conserved regions (U1-U4, red); conserved region plant (CR-P, green); and class-specific
region(CSR,purple). Thediagramsare toscale,with50aminoacids includedinthe length
indicatedintheKEY(upperright).
plant vs., for example, bacterial CSCs, i.e., assembly into 'rosette' geometry, co-assembly
with other isoforms, interaction with other proteins in the plant cell cortex, and movement
in an oriented manner within the plasma membrane.
Mutant analysis has shown that several other proteins are required for cellulose bio-
genesis, as judged by cellulose deficiencies and/or arrested or abnormal development in
mutants of Arabidopsis and a few other species. Several of these, including Korrigan,
the COBRA-type proteins, and Kobito, are membrane spanning proteins that may be
integrated within or near the CSC (12). This possibility exists even for proteins not
yet shown to exist in the plasma membrane by fluorescence technology, which may not
resolve proteins integrated within the membrane in groups of fewer than 10 molecules
(13). Other proteins not yet identified may also function along with the CSC to accom-
plish cellulose biogenesis. In the discussion below, we will emphasize CesA since it is
the only proven component of the plant CSC.
2.4
The Functional Operation of the CSC
In the absence of a real mechanistic understanding or even identification of all of the
components, we expect the plant CSC to carry out several activities. As the CSC
mediates cellulose biogenesis to build the cell wall it must: assemble with genetically
determined morphology; stabilize in operational form in the plasma membrane; acquire
UDP-glucose substrate; polymerize glucose with ß-1,4-linkage; operate so that fibrils
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