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to the phosphorylation and dephosphorylation of plasmodesmal proteins. In the case
of selective gating, each MP would directly interact with plasmodesmal proteins,
'relaxing' the putative myosin spokes bound to the plasma membrane, and enabling
modulation of the SEL (Schulz, 1999). However, it is not known whether all endoge-
nous macromolecular proteins that are selectively trafficked through plasmodesmata
require phosphorylation to mediate their passage. Oparka (2004) has proposed a hy-
pothetical model for the role of phosphorylation and dephosphorylation in protein
movement through plasmodesmata. In this model the trafficked protein is bound to
a motor protein by a specific chaperone for delivery to plasmodesmata. At the pore,
the complex interacts with the putative 'docking' protein. Recognition of a traffick-
ing motif on the protein cargo activates a myosin-specific kinase that phosphorylates
the C-terminus of the myosin motor, releasing it from the plasma membrane. The
released myosin, together with its cargo, then moves along the actin filaments as-
sociated with the desmotubule via its motor domain. Oparka (2004) proposes that
phosphorylation and dephosphorylation of the motor protein might regulate the de-
tachment and attachment of the protein cargo from the plasma membrane lining the
plasmodesmal pore.
Haywood
et al.
(2002) have also suggested that other forms of protein structural
modification may be involved in controlling selective transport through plasmodes-
mata. They cite the example of the phloem protein CmPP36, which is able to induce
only an increase in SEL and move from cell to cell in an N-terminally truncated
form, indicating that the capacity of this NCAP to target to and/or transport through
plasmodesmata is controlled by proteolytic processing (Xoconostle-Cazares
et al.
,
2000).
In certain cases, selective gating may be achieved by conformational changes in
the trafficking protein or macromolecular complex. Kragler
et al.
(1998) demon-
strated that partial unfolding of KN1 appears to be involved during its translocation
through plasmodesmata. Yeast two-hybrid screens using the putative MP of
Tomato
spotted wilt virus
(TSWV) as bait identified two DnaJ-like interacting proteins from
Nictoiana tabacum
and
Arabidopsis
(Soellick
et al.
, 2000) and another DnaJ-like
protein from
Lycopersicon esculentum
(von Bargen
et al.
, 2001). DnaJ proteins be-
long to the Hsp40 subclass of heat-shock proteins, and are known to be involved
in protein import into organelles and in the regulation of the chaperone, Hsp70
(Kelley, 1999). Both Jackson (2001) and Blackman and Overall (2001) have sug-
gested that chaperone activity may be involved in cell-to-cell trafficking of MPs, and
could have a role in partially unfolding proteins for translocation through the pore
(Kragler
et al.
, 1998). Alternatively, it may function directly as a motor to facilitate
translocation through the plasmodesmal pore (Alzhanova
et al.
, 2001). Aoki
et al.
(2002) detected Hsc70 (Hsp70-related) proteins in a plasmodesmal-enriched cell
wall fraction and isolated and characterised two Hsp70 chaperones from
C. maxima
phloem sap. Using mutational analysis, Aoki
et al.
(2002) showed these proteins
contain a motif that mediates their intercellular trafficking. Fusion of this motif to
a human Hsp70 chaperone allowed the latter to move from cell to cell. Aoki
et al.
(2002) have postulated that these pumpkin Hsp70 chaperones may play a role in
