Arabidopsis Kinetochore Fiber-Associated MAP65-4 Cross-Links Microtubules and Promotes Microtubule Bundle Elongation

Vincent Fache,Daniel Van Damme,Virginie Stoppin-Mellet,Jérémie Gaillard,Danny Geelen,Emmanuelle Neumann,Marylin Vantard

doi:10.1105/tpc.110.080606

Abstract

The acentrosomal plant mitotic spindle is uniquely structured in that it lacks opposing centrosomes at its poles and is equipped with a connective preprophase band that regulates the spatial framework for spindle orientation and mobility. These features are supported by specialized microtubule-associated proteins and motors. Here, we show that Arabidopsis thaliana MAP65-4, a non-motor microtubule associated protein (MAP) that belongs to the evolutionarily conserved MAP65 family, specifically associates with the forming mitotic spindle during prophase and with the kinetochore fibers from prometaphase to the end of anaphase. In vitro, MAP65-4 induces microtubule (MT) bundling through the formation of cross-bridges between adjacent MTs both in polar and antipolar orientations. The association of MAP65-4 with an MT bundle is concomitant with its elongation. Furthermore, MAP65-4 modulates the MT dynamic instability parameters of individual MTs within a bundle, mainly by decreasing the frequency of catastrophes and increasing the frequency of rescue events, and thereby supports the progressive lengthening of MT bundles over time. These properties are in line with its role of initiating kinetochore fibers during prospindle formation.

Highlights

In plant cells, microtubule (MT) cytoskeleton arrays provide a molecular framework for various cellular processes, including cell morphogenesis, establishment of cell polarity, and cell division (Ehrhardt, 2008)
Error bars indicate SD. (B) Catastrophe frequencies measured at the minus and plus ends of MT bundled by different MAP65-4 concentrations
MAP65-4 decreases the frequency of catastrophe and rescue events at both MT ends. (C) Rescue frequencies measured at the MT minus and plus ends in the presence of various concentrations of MAP65-4

Summary

Introduction

Microtubule (MT) cytoskeleton arrays provide a molecular framework for various cellular processes, including cell morphogenesis, establishment of cell polarity, and cell division (Ehrhardt, 2008). Higher plant MT arrays do not emanate as radial arrays from a well-defined MT-organizing center (i.e., the centrosome). Instead, they are composed mainly of MT bundles, which are highly dispersed within the cell cortex during interphase (and known as cortical MTs) or organized as a mitotic spindle during cell division. Plant cells organize a ring of MT bundles at the onset of mitosis, called the preprophase band (PPB). This PPB defines the future division plane and the position of interdigitated MT bundles at the midzone during cytokinesis, a structure referred to as the phragmoplast. The bundling of MTs is a crucial step in the formation and stabilization of MT arrays throughout the cell cycle and hinges on the ability of

Methods

Results

Conclusion