48 A.J. Wright · L.G. Smith
3.3.1
Tangled, POK1, and POK2
Recently, the first positive marker of the division site has been discovered.
Thetangled1(tan1) gene was originally studied in maize, where mutations
in this gene cause a high frequency of misoriented cell divisions (Smith et al.
1996). This defect was attributed mainly to the failure of most phragmoplasts
to be guided to former PPB sites, suggesting a role for TAN1 in division plane
establishment and/or phragmoplast guidance (Cleary and Smith 1998).Ta n 1
was cloned and found to encode a highly basic protein weakly similar to
MT binding domains of vertebrate adenomatous polyposis coli (APC) pro-
teins, and in vitro assays showed that TAN1 is capable of binding MTs (Smith
et al. 2001).tan1is expressed in tissues where cells are actively dividing, but
a specific protein localization could not be determined since anti-TAN1 an-
tibodies apparently cross-reacted with other, TAN1-related proteins (Smith
et al. 2001).
To overcome this difficulty, the localization pattern of theArabidopsis
TA N 1 h o m o l o g ( At TA N ) w a s d e t e r m i n e d by f u s i n g i t t o Y F P. S t a r t l i n g l y,
AtTAN localizes to a ring coincident with the PPB in preprophase/prophase
cells, which remains in position throughout mitosis and cytokinesis suggest-
ing that AtTAN serves as a positive memory of the division site after the PPB
breaksdown(WalkerK,EhrhardtD,SmithLG,unpublished).Confirmingan
important role for AtTAN in spatial control of cytokinesis, root tips of plants
with mutations in this gene have misoriented divisions attributable to mis-
guided phragmoplasts. While maintenance of the AtTAN ring was found to
be MT independent,ton2mutants, which lack PPBs, also lack AtTAN rings
(Walker K, Ehrhardt D, Smith LG, unpublished).
PHRAGMOPLAST ORIENTING KINESIN 1 and 2 (POK1 and POK2) were
originally identified in maize as interactors with TAN1 via a yeast two-hybrid
screen (Müller et al. 2006).Arabidopsismutants for both genes were iso-
lated, and while neither single mutant has a phenotype, thepok1;2double
mutant is dwarfed with misoriented planes of cell division suggesting a role
for these kinesins in the orientation of the plane of division (Müller et al.
2006). As in maizetan1mutants, phragmoplasts often fail to be guided to for-
mer PPB sites inpok1;2double mutants. Additionally,POK1/2are needed for
effective recruitment of AtTAN to the site of the PPB suggesting that the func-
tionally redundant motor activities of POK1 and POK2 are used to localize
AtTAN during preprophase (Muller S, Ehrhardt D, Smith LG, unpublished).
Since TON2, the putative phosphatase regulatory subunit, is also required for
AtTAN localization this suggests that the phosphorylation status of AtTAN,
POK1, or POK2 may be relevant to the formation of the AtTAN ring. Alter-
natively, the dependence of AtTAN ring formation on TON2 maybe related
to its role in PPB formation via hypothesized dephosphorylation of proteins
needed for MT stability as previously discussed (Sect. 2.2.2).