Transportin regulates major mitotic assembly events: from spindle to nuclear pore assembly.
Mitosis in higher eukaryotes is marked by the sequential assembly of two massive structures: the mitotic spindle and the nucleus. Nuclear assembly itself requires the precise formation of both nuclear membranes and nuclear pore complexes. Previously, importin alpha/beta and RanGTP were shown to act as dueling regulators to ensure that these assembly processes occur only in the vicinity of the mitotic chromosomes. We now find that the distantly related karyopherin, transportin, negatively regulates nuclear envelope fusion and nuclear pore assembly in Xenopus egg extracts. We show that transportin-and importin beta-initiate their regulation as early as the first known step of nuclear pore assembly: recruitment of the critical pore-targeting nucleoporin ELYS/MEL-28 to chromatin. Indeed, each karyopherin can interact directly with ELYS. We further define the nucleoporin subunit targets for transportin and importin beta and find them to be largely the same: ELYS, the Nup107/160 complex, Nup53, and the FG nucleoporins. Equally importantly, we find that transportin negatively regulates mitotic spindle assembly. These negative regulatory events are counteracted by RanGTP. We conclude that the interplay of the two negative regulators, transportin and importin beta, along with the positive regulator RanGTP, allows precise choreography of multiple cell cycle assembly events.
Lau
Corine K
CK
Section of Cell and Developmental Biology, Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093-0347, USA.
Delmar
Valerie A
VA
Chan
Rene C
RC
Phung
Quang
Q
Bernis
Cyril
C
Fichtman
Boris
B
Rasala
Beth A
BA
Forbes
Douglass J
DJ
D000595:Amino Acid Sequence; D000818:Animals; D002843:Chromatin / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D028884:Karyopherins / Q000378:metabolism*; D008561:Membrane Fusion; D008938:Mitosis; D008954:Models, Biological; D008969:Molecular Sequence Data; D022022:Nuclear Pore / Q000378:metabolism*; D028861:Nuclear Pore Complex Proteins / Q000378:metabolism; D011485:Protein Binding; D021381:Protein Transport; D008941:Spindle Apparatus / Q000378:metabolism*; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism; D014981:Xenopus; D029867:Xenopus Proteins / Q000737:chemistry / Q000378:metabolism; D028961:beta Karyopherins / Q000378:metabolism; D020931:ran GTP-Binding Protein / Q000378:metabolism
C525081:AHCTF1 protein, Xenopus; D002843:Chromatin; D004268:DNA-Binding Proteins; D028884:Karyopherins; D028861:Nuclear Pore Complex Proteins; D014157:Transcription Factors; D029867:Xenopus Proteins; D028961:beta Karyopherins; D020931:ran GTP-Binding Protein