Crosslinkers and Motors Organize Dynamic Microtubules to Form Stable Bipolar Arrays in Fission Yeast

Abstract

Microtubule (MT) nucleation not only occurs from centrosomes, but also in large part from dispersed nucleation sites. The subsequent sorting of short MTs into networks like the mitotic spindle requires molecular motors that laterally slide overlapping MTs and bundling proteins that statically connect MTs. How bundling proteins interfere with MT sliding is unclear. In bipolar MT bundles in fission yeast, we found that the bundler ase1p localized all along the length of antiparallel MTs, whereas the motor klp2p (kinesin-14) accumulated only at MT plus ends. Consequently, sliding forces could only overcome resistant bundling forces for short, newly nucleated MTs, which were transported to their correct position within bundles. Ase1p thus regulated sliding forces based on polarity and overlap length, and computer simulations showed these mechanisms to be sufficient to generate stable bipolar bundles. By combining motor and bundling proteins, cells can thus dynamically organize stable regions of overlap between cytoskeletal filaments.

Two Microtubule-associated Proteins of Arabidopsis MAP65s Promote Antiparallel Microtubule Bundling

“…During late metaphase and anaphase AtMAP65-5 localizes to the midzone and AtMAP65-1 during late anaphase where numerous nonkinetochore MTs are antiparallel overlapping MT plus ends. Noticeably, Ase1p has the inherent ability to distinguish between parallel and antiparallel MTs (Janson et al, 2007). We propose a similar property for AtMAP65-1 and AtMAP65-5 but their ability to control the self-organization of MT bundles in plant cells remain to be addressed.…”

“…In fluorescence assays, AtMAP65-1 and AtMAP65-5 accumulated along MT bundles but not along single MTs ( Figure 4C). In comparison, in fission yeast binding of Ase1p to MTs is reinforced when Ase1p binds overlapped MTs (Janson et al, 2007). In vivo, Van Damme et al (2004a) reported that AtMAP65-1-GFP or AtMAP65-5-GFP associated with coaligned MTs and suggested that AtMAP65-1-GFP may not bind to single MTs.…”

“…Janson et al (2007) demonstrated that Ase1p localizes to overlapping (Ϫ) ends during interphase and overlapping plus ends in the anaphase spindle, the difference in localization being determined by molecular motors as Klp2p (a minus-end-directed motor) mediates the sliding of newly nucleated MTs along antiparallel MT bundles. In plant cells, ATK5, a plus end tracking protein molecular motor (Ambrose et al, 2005) preferentially localizes at regions of overlap between opposing interpolar MTs, suggesting an increased affinity for plus ends in an antiparallel orientation (Ambrose and Cyr, 2007).…”

“…They are mainly members of the MT-associated family MAP65s (Smertenko et al, 2004;Wicker-Planquart et al, 2004;Li et al, 2007a). MAP65 proteins are evolutionarily conserved, nonmotor microtubule-associated proteins (MAPs) that in animal and yeast cells accumulate in the spindle midzone during late anaphase to stabilize overlapping antiparallel arranged MTs (Verni et al, 2004;Zhu et al, 2006;Janson et al, 2007). In Arabidopsis nine AtMAP65 family members were identified (Hussey et al, 2002).…”

PRC 1‐labeled microtubule bundles and kinetochore pairs show one‐to‐one association in metaphase

“…Orthologs of PRC1 with conserved function include Ase1 (anaphase spindle elongation 1) in yeasts 14, 16, SPD‐1 (spindle defective 1) in Caenorhabditis elegans 17, Feo (Fascetto) in Drosophila melanogaster 18, and MAP65 (microtubule‐associated protein 65) in plants 19, all of which fall in a conserved family of non‐motor microtubule‐associated proteins (MAPs).…”