130 T. Sano et al.
MTOC. Recently, Murata et al. (2005) demonstrated that in tobacco BY-GT
cells, MTs were nucleated as branches on the extant cortical MTs. In add-
ition, at the branch points,γ-tubulin was localized to and functioned in MT
nucleation. These observations suggest that in higher plant cells the MTs nu-
cleate from theγ-tubulin complex, the localization of which is not limited to
distinct structures but is rather broadly associated with MTs. The MT branch-
ing system based on theγ-tubulin complex is also involved in the expansion
of phragmoplast MTs (T. Murata, personal communication). InArabidopsis,
depletion ofγ-tubulin proteins by RNA interference or by gene disruption
resulted in aberrant spindle and phragmoplast structures and affected cytoki-
nesis (Binarová et al. 2006; Pastuglia et al. 2006), confirming the involvement
ofγ-tubulin in cell division through MT nucleation activity.
MAPs.Microtubule associated proteins (MAPs) play potential roles in the
regulation of MT organization and function during cell cycle progression.
Numerous proteins either with MT binding activity or related to proteins with
MT binding activity have been identified to date, and herein, MAP65 and
MOR1, which were recently cloned and biochemically characterized (Hussey
et al. 2002) are described below. Readers can refer a recent review concerning
plant MAPs (Hamada 2007).
MAP65wasfirstbiochemicallypurifiedfromtobaccoBY-2cellsby
the polymerization and depolymerization procedure of tubulin (Jiang and
Sonobe 1993). The 65 kDa protein shows MT-bundling activity and forms
cross-bridge structures between adjacent MTs in vitro, as confirmed by the
Arabidopsis AtMAP65-1recombinant protein (Smertenko et al. 2004). Al-
though the deduced amino acid sequence of the corresponding cDNA shows
only a 20 % homology withSaccharomyces cerevisiaeAse1 (anaphase spindle
elongation factor) and the vertebrate PRC1 (protein regulating cytokinesis),
the plant MAP65 is now proposed to be a functional equivalent of these pro-
teins since all three proteins are components of mitotic spindles and their
expression or/and localization is cell cycle dependent (Hussey et al. 2002;
Mollinari et al. 2002; Schuyler et al. 2003). Immuno-fluorescence microscopy
and studies with GFP fusion proteins have further revealed an overlap be-
tween the localization of MAP65 and regions of MTs, namely the PPB, mitotic
spindle and phragmoplast in addition to CMTs (Smertenko et al. 2000, Mao
et al. 2005), suggesting a role for MAP65 in events involving cell cycle pro-
gression. Completion of theArabidopsis thalianagenome sequencing project
identified nineArabidopsisMAP65 genes (Hussey et al. 2002) with deduced
amino acid sequences showing 28 – 79 % identity and predicted molecular
masses varying from 54 to 80 kDa. Although characterization of these plant
MAP65 proteins is still incomplete, it is possible that the proteins play diver-
gent roles in the regulation of MT organization and activity in plant cells.
MOR1 was initially identified in a screen ofArabidopsismutants with
aberrant microtubule patterns (Whittington et al. 2001). One mutant,mor1,
showed temperature-sensitive cortical microtubule shortening and disor-