Title: A novel "volume-sensing" mechanism for the spatial-
organization of the cytoskeleton.

Abstract: Fragments of fish pigment cells can form and center
aggregates of pigment granules by dynein-motor-driven transport along
a self-organized radial array of microtubules (MTs). I will present a
quantitative model that describes pigment aggregation and MT-aster
self-organization and the subsequent centering of both structures.
The model is based on the observations that MTs are immobile and
treadmill, while dynein-motor-covered granules have the ability to
nucleate MTs. From assumptions based on experimental observations,
I'll derive partial integro-differential equations describing the
coupled granule-MT interaction. Scaling arguments and perturbation
theory allow for analysis in two limiting cases. This analysis
explains the mechanism of aster self-organization as a positive
feedback loop between motor aggregation at the MT minus ends and MT
nucleation by motors. Furthermore, the centering mechanism is
explained as a global geometric bias in the cell established by
spontaneously-nucleated microtubules. Numerical simulations lend
additional support to the analysis. The model sheds light on role of
polymer dynamics and polymer-motor interactions in cytoskeletal
organization.