150 J.C. Ambrose · R. Cyr
and Smith 1998). Although transverse PPBs appear normal, the occurrence of
longitudinal PPBs is markedly reduced, suggesting a role for Tangled in di-
vision plane determination. Although Tan1 has been localized to the mitotic
spindle (Smith et al. 2001), its role there remains to be determined.
Spiral.TheArabidopsis Spiral(SPR1) gene product is a novel 12 kD plant-
specific MAP belonging to a family with five otherSPR-like genes. First
identified in screens for mutants with skewed root growth,spr1plants exhibit
right-handed twisting of organs (Furutani et al. 2000; Sedbrook et al. 2004).
SPR1::GFP localizes to all MT arrays and is a +TIP (Nakajima et al. 2004; Sed-
brook et al. 2004). Although localized in mitotic spindles, its function also
remains to be determined (Sedbrook et al. 2004).
Map70.Given the complex organization and rearrangements of plant MT
arrays, a large toolbox of novel MAPs may exist that would not be identified
by homology searches using known animal MAPs. The identification of the
novelArabidopsisprotein MAP70 through proteomic screens for MT-binding
proteins is one such example (Korolev et al. 2005). Identified in proteomic
screens of proteins bound totaxol-stabilized MTs inArabidopsissuspension
cultures, MAP70 is a novel plant-specific coiled-coil protein with 5Arabidop-
sisfamily members. GFP fusions to MAP70 decorate all 4 arrays (Korolev et al.
2005), although its function remains to be identified.
Map190. MAP190 is a novel plant MAP isolated from BY-2 cells based on
its affinity for both MTs and MFs; and it localizes to the nucleus in inter-
phase and to the spindle during mitosis (Igarashi et al. 2000). The presence
of a MAP with actin-binding capacity within mitotic spindles suggests a pos-
sible role for actin in mitosis. Actin has been found within spindles and has
been implicated in spindle positioning and orientation, but its contribution to
spindle function, if any, is questionable (Mineyuki and Palevitz 1990; Staiger
and Cande 1991).
3.2
Kinesins: Force-Generating MAPs
Kinesins are force-generating MAPs that use the hydrolysis of ATP to move
unidirectionally along microtubules, carrying cellular cargo such as or-
ganelles, vesicles, and chromosomes (Dagenbach and Endow 2004; Miki et al.
2005). A standardized nomenclature was developed that recognizes 14 dis-
tinct kinesin families (Lawrence et al. 2004). This nomenclature will be used
throughout this work. Because of the conserved nature of kinesin function
within the spindle apparatus, it is helpful to give a short overview of the
known functional data available from non-plant studies before describing
plant mitotic kinesins.
Kinesins involved in spindle function.At least eight kinesin families con-
tain members that are involved in some aspect of cell division (Goshima
and Vale 2003; Zhu et al. 2005). Of these, five families appear to facilitate