Biomedical Engineering Reference
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Figure2.1 Transmissionelectronmicrographof rosetteCSCswithintheplasmamembrane
of Zinnia elegans cells that were depositing secondary walls during tracheary element
differentiationin suspensionculture. The specimenwas preparedby cryo-fracture followed
by shadowing with platinum and carbon, and the inner leaflet of the cleaved plasma
membraneisshown. Thesixsubunitsof therosetteCSCs(circled)reflectaggregatesofCesA
transmembranehelicaldomainstraversingthemembrane.TheCSCsinthemainmicrograph
reflectthetypicalmaximumdensityseenduringsecondarywalldepositioninprimaryxylem
elements. The inset in the upper left shows one CSC at higher magnification. The main
micrographandtheinsetshow,respectively,theresultsofunidirectionalorrotaryshadowing
by platinum/carbon. The bars in the main micrograph and the inset represent 30 nm and
10nm,respectively.
predicted N- and C-terminal transmembrane domains (TMD) where 2-6 TMHs pass
through the membrane. In total, there are 8-9 TMH in CS proteins, with fewer near
the N-terminus, more at the C-terminus, and a long intervening hydrophilic region (the
catalytic region). The U1-U4 regions are 5-7 amino acids in length with 70-80% amino
acid conservation in all species; these are involved in the catalytic function (see below).
Of the examples in Figure 2.2, only G. hirsutum is known to have rosette CSCs: A. vari-
abilis has an unknown CSC structure; the D. discoideum CSC can exist in linear or block
like arrays; and the G. xylinus CSC forms an extended linear array. Correspondingly,
only the plant-type CESA proteins, e.g. from G. hirsutum , are known to include an
N-terminal, cysteine-rich, zinc-binding domain (ZnBD) and a variable 'class specific
region' (CSR) between U2 and U3 (8). The sequence of the insertion between the U1
and U2 domains (designated the CR-P, for 'conserved region-plant') is also unique to
plant-type CesAs (9). Although other CS proteins from D. discoideum (10) and certain
cyanobacteria such as A. variabilis (11) have insertions in the same area, these are not
clearly homologous to the CR-P region.
The known or possible functions of these protein domains will be discussed further
below. In general, the absence of the ZnBD and the CR-P and CSR regions in all
non-plant CS proteins demonstrates that they are not needed to carry out ß-1,4-glucan
polymerization.
Instead, they could modulate the additional functions associated with
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