Abstract:
The transport of dynamic microtubules (MTs) driven by dyneins is essential for spindle
assembly, asymmetric cell division, axonal growth and organelle positioning. While the
single-molecule mechano-chemistry of dyneins has advanced extensively, their role in
collective MT transport, remains unclear. Additionally, MTs in animal somatic cells are
typically nucleated by centrosomes forming radial arrays or asters. MT aster transport
in oocytes and single-cell embryos involves a tug-of-war of dyneins that is resolved
by a combination of MT polymerization dynamics regulation, motor localization and
self-organized clustering. This work encompasses (a) the length and motor number
dependence of microtubule transport by yeast dyneins, (b) aster-based transport of nuclei
in Saccharomyces cerevisiae and (c) compares polymerization dynamics of MTs from
plant and animal sources. The work shows that microtubules transported by teams of
immobilized S. cerevisiae dynein transits from random to directed motion with increasing
MT lengths and motor densities, both in vitro and in silico. Velocity, diffusivity and
directionality, all reflect a coordination of transport above a threshold motor numbers.
Our analysis of in vivo data of astral MTs transported by dynein during S. cerevisiae
mitosis supports a functional role for such a transition. Based on these findings, a general
‘search and orient’ mechanism is proposed for organelle positioning by MT asters.
The comparative study on the role of MT polymerization dynamics from multiple sources:
goat, porcine and plant (mung beans) demonstrates nucleation limited polymerization in
all these diverse species, suggesting a central role of nucleators such as centrosomes in
determining MT filament distributions. The model proposed highlights the importance of
critical concentration in determining filament length distributions. Optimizations with ex
vivo reconstitutions of nucleators such centrosomes and spindle pole body will advance
our understanding of the collective effects of motor numbers, MT length dynamics and
transport of complex geometry in eukaryotic cells.
