TY - JOUR
T1 - Transportin acts to regulate mitotic assembly events by target binding rather than Ran sequestration
AU - Bernis, Cyril
AU - Swift-Taylor, Beth
AU - Nord, Matthew
AU - Carmona, Sarah
AU - Chook, Yuh Min
AU - Forbes, Douglass J.
PY - 2014/4/1
Y1 - 2014/4/1
N2 - The nuclear import receptors importin ß and transportin play a different role in mitosis: both act phenotypically as spatial regulators to ensure that mitotic spindle, nuclear membrane, and nuclear pore assembly occur exclusively around chromatin. Importin ß is known to act by repressing assembly factors in regions distant from chromatin, whereas RanGTP produced on chromatin frees factors from importin ß for localized assembly. The mechanism of transportin regulation was unknown. Diametrically opposed models for transportin action are as follows: 1) indirect action by RanGTP sequestration, thus down-regulating release of assembly factors from importin ß, and 2) direct action by transportin binding and inhibiting assembly factors. Experiments in Xenopus assembly extracts with M9M, a superaffinity nuclear localization sequence that displaces cargoes bound by transportin, or TLB, a mutant transportin that can bind cargo and RanGTP simultaneously, support direct inhibition. Consistently, simple addition of M9M to mitotic cytosol induces microtubule aster assembly. ELYS and the nucleoporin 107-160 complex, components of mitotic kinetochores and nuclear pores, are blocked from binding to kinetochores in vitro by transportin, a block reversible by M9M. In vivo, 30% of M9M-transfected cells have spindle/cytokinesis defects. We conclude that the cell contains importin ß and transportin "global positioning system"or "GPS" pathways that are mechanistically parallel.
AB - The nuclear import receptors importin ß and transportin play a different role in mitosis: both act phenotypically as spatial regulators to ensure that mitotic spindle, nuclear membrane, and nuclear pore assembly occur exclusively around chromatin. Importin ß is known to act by repressing assembly factors in regions distant from chromatin, whereas RanGTP produced on chromatin frees factors from importin ß for localized assembly. The mechanism of transportin regulation was unknown. Diametrically opposed models for transportin action are as follows: 1) indirect action by RanGTP sequestration, thus down-regulating release of assembly factors from importin ß, and 2) direct action by transportin binding and inhibiting assembly factors. Experiments in Xenopus assembly extracts with M9M, a superaffinity nuclear localization sequence that displaces cargoes bound by transportin, or TLB, a mutant transportin that can bind cargo and RanGTP simultaneously, support direct inhibition. Consistently, simple addition of M9M to mitotic cytosol induces microtubule aster assembly. ELYS and the nucleoporin 107-160 complex, components of mitotic kinetochores and nuclear pores, are blocked from binding to kinetochores in vitro by transportin, a block reversible by M9M. In vivo, 30% of M9M-transfected cells have spindle/cytokinesis defects. We conclude that the cell contains importin ß and transportin "global positioning system"or "GPS" pathways that are mechanistically parallel.
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U2 - 10.1091/mbc.E13-08-0506
DO - 10.1091/mbc.E13-08-0506
M3 - Article
C2 - 24478460
AN - SCOPUS:84898756024
SN - 1059-1524
VL - 25
SP - 992
EP - 1009
JO - Molecular biology of the cell
JF - Molecular biology of the cell
IS - 7
ER -