52 A.J. Wright · L.G. Smith
MTs connecting the phragmoplast/nuclear complex to the cell cortex (re-
ferred to as “astral MTs” or “endoplasmic MTs”) have been proposed to play
a role in orienting the expanding phragmoplast, although evidence of the
functional importance of this MT population has not yet been reported (Chan
et al. 2005; Dhonukshe et al. 2005b). Certainly, this is an attractive idea, par-
ticularly given the lack of evidence that MFs can perform this function, and
the well-established role of astral MTs in orienting the mitotic spindle via
interactions with the cell cortex in animal cells, where spindle orientation de-
termines the plane of division (cleavage). Interestingly, one mechanism for
spindle orientation in animal cells involves a direct interaction of EB1 at MT
plus ends with local, cortical accumulations of adenomatous polyposis coli
(APC) protein (Lu et al. 2001; McCartney et al. 2001; Yamashita et al. 2003).
The TANGLED protein, which is localized at the former PPB site in the cell
cortex throughout mitosis and cytokinesis (Walker K, Ehrhardt D, Smith LG,
unpublished), is distantly related to the basic domain of APC, though it lacks
EB1-binding and other domains (Smith et al. 2001). Nonetheless, an intrigu-
ing model for future investigation is that EB1 at the plus ends of MTs radiating
from the phragmoplast/nuclear complex interact (perhaps via an unknown
linker protein) with TAN at the division site to help orient phragmoplast ex-
pansion, thereby aiding attachment of the cell plate at the former PPB site.
5
Conclusions
While the roles of MTs and MFs in the process of cell division generally and
the orientation of the plane of division specifically are becoming ever more
clear, there is still relatively little known about the identity and function of
other proteins that participate in specifying the orientation of cell division.
The challenge for the future will be to discover the proteins that regulate the
formation and positioning of the PPB/phragmosome, spindle, and phragmo-
plast and determine how these proteins interpret polarity, external cues, and
cell geometry to specify the correct positioning of cytoskeletal structures gov-
erning cell plate orientation.
AcknowledgementsWork on cytokinesis in the laboratory of LGS was supported by NIH
R01 GM53137 and AJW was supported by NIH GM68524 (UCSD/SDSU IRACDA).
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