Nucleocytoplasmic Shuttling of the TACC Protein Mia1p/Alp7p Is Required for Remodeling of Microtubule Arrays during the Cell Cycle

Kevin G Hardwick,Yuen Chyao Ling,Aleksandar Vjestica,Snezhana Oliferenko

doi:10.1371/journal.pone.0006255

Abstract

Microtubule arrays are remodeled as cells proceed through the cell cycle. It is important to understand how remodeling is regulated in time and space. In fission yeast, the conserved microtubule associated TACC/TOG complex plays an important role in organizing microtubules throughout the cell cycle. Here we show that this complex undergoes nucleocytoplasmic shuttling through the nuclear import and export signals located in the TACC protein Mia1p/Alp7p. When the Crm1p-dependent nuclear export signal of Mia1p is disabled, Mia1p accumulates in the nucleus while its partner protein Alp14p/TOG is restricted to the cytoplasm. This leads to defects in assembly of both interphase arrays and the mitotic spindle. Artificial targeting of Alp14p to the nucleus partially rescues the mitotic spindle defects caused by lack of Mia1p nuclear export. Interestingly, the nuclear export sequence of Mia1p appears to overlap with the Alp14p binding site. We propose that intricate regulation of the subcellular distribution of TACC/TOG complexes drives microtubule array remodeling as cells progress through the cell cycle.

Highlights

Microtubules are dynamic polymers that often function as higher order arrays of different geometries that form in response to cell cycle and environmental cues
In ‘‘open’’ mitosis of higher eukaryotes the nuclear envelope (NE) breaks down enabling microtubules to capture kinetochores. In many organisms, such as fission yeast Schizosaccharomyces pombe (S. pombe), the NE stays intact so spindles are assembled from tubulin and microtubule-associated proteins (MAPs) that are imported from the cytoplasm through the nuclear pore complexes
While nuclear localization of large proteins and protein complexes requires specialized nuclear import sequence (NLS), at least two possibilities may account for the redistribution of MAPs once mitosis is complete

Summary

Introduction

Microtubules are dynamic polymers that often function as higher order arrays of different geometries that form in response to cell cycle and environmental cues. In ‘‘open’’ mitosis of higher eukaryotes the nuclear envelope (NE) breaks down enabling microtubules to capture kinetochores In many organisms, such as fission yeast Schizosaccharomyces pombe (S. pombe), the NE stays intact so spindles are assembled from tubulin and microtubule-associated proteins (MAPs) that are imported from the cytoplasm through the nuclear pore complexes. In S. pombe, Ran-GTP promotes nuclear accumulation of an evolutionary conserved MAP complex that consists of the transforming acidic coiled coil (TACC) protein Mia1p/Alp7p (which appears to be a direct Ran target) and the TOG protein Alp14p [6] It is unclear whether nucleocytoplasmic shuttling occurs throughout the cell cycle and why this complex accumulates in the nucleus only at mitotic onset. Our results underscore the importance of spatiotemporal regulation of the activity and availability of MAPs for proper microtubule array formation

Methods

Results

Conclusion