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refolding structurally altered proteins so that they can enter the sieve elements from
the companion cell.
Recently, NCAPP1 (non-cell autonomous pathway protein 1) was isolated from
a plasmodesmal-enriched cell wall protein fraction using the CmPP16 trafficking
protein as bait for candidate components of the plasmodesmal translocation ma-
chinery (Lee
et al.
, 2003). Microinjection studies of a truncated version of NCAPP1
indicated that NCAPP1 plays a role in mediating the cell-to-cell trafficking of selec-
tive endogenous proteins. Fluorescent protein tagging, coupled with immunogold
localisation, confirmed its localisation to the peripheral endoplasmic reticulum, ap-
proximately 200 nm away from the pore itself. Lee
et al.
(2003) proposed that
NCAPP1 is anchored to the endoplasmic reticulum membrane, with the bulk of
the protein exposed in the cytoplasm where it interacts with proteins moving to-
wards the plasmodesmal pore. Although NCAPP1 was found to be obligatory for
the movement of CmPP16 and TMV MP, other macromolecular proteins, including
KN1 and CMV MP, were able to traffic without NCAPP1, suggesting that NCPP1
does not play a role in the trafficking of these other NCAPs (Jackson & Kim,
2003).
5.3.7 The emerging picture of plasmodesmata
Current research on plasmodesmata has indicated that macromolecular trafficking
between plant cells may be a commonplace event. Plasmodesmata can no longer be
viewed as simple channels through the plant cell wall that facilitate the movement of
nutrients. Instead, they have a much more complex role in controlling intercellular
communication and the flux of information passing through the symplasm. In the
future, it will be necessary to examine the ways in which proteins and ribonucleopro-
tein complexes interact with components of the plasmodesmal pore. To do this will
require a systematic isolation and characterisation of the genes that encode novel
plasmodesmal components, and an understanding of the mechanism by which these
integral proteins interact with the various 'cargos' that are being shuttled between
plant cells. The genetics of plasmodesmata is in its infancy and there is a pressing
need to identify the gene products that regulate the formation and development of
plasmodesmata. Manipulation of plasmodesmal development is an exciting target
for the future and should lead to a more comprehensive picture of how cells com-
municate with one another, as well as determining the internal and external forces
that impinge on this process.
Acknowledgements
The authors are supported by grants awarded by the Scottish Executive Environment
and Rural Affairs Department (SEERAD) and the Gatsby Foundation. The authors
thank Prof. Chris Hawes for providing Fig. 5.3.
