Importin-beta and the small guanosine triphosphatase Ran mediate chromosome loading of the human chromokinesin Kid |
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Authors: | Tahara Kiyoshi Takagi Masatoshi Ohsugi Miho Sone Takefumi Nishiumi Fumiko Maeshima Kazuhiro Horiuchi Yasuomi Tokai-Nishizumi Noriko Imamoto Fumio Yamamoto Tadashi Kose Shingo Imamoto Naoko |
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Affiliation: | Cellular Dynamics Laboratory, Discovery Research Institute, Institute of Physical and Chemical Research, Wako, Saitama, 351-0198, Japan. |
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Abstract: | Nucleocytoplasmic transport factors mediate various cellular processes, including nuclear transport, spindle assembly, and nuclear envelope/pore formation. In this paper, we identify the chromokinesin human kinesin-like DNA binding protein (hKid) as an import cargo of the importin-alpha/beta transport pathway and determine its nuclear localization signals (NLSs). Upon the loss of its functional NLSs, hKid exhibited reduced interactions with the mitotic chromosomes of living cells. In digitonin-permeabilized mitotic cells, hKid was bound only to the spindle and not to the chromosomes themselves. Surprisingly, hKid bound to importin-alpha/beta was efficiently targeted to mitotic chromosomes. The addition of Ran-guanosine diphosphate and an energy source, which generates Ran-guanosine triphosphate (GTP) locally at mitotic chromosomes, enhanced the importin-beta-mediated chromosome loading of hKid. Our results indicate that the association of importin-beta and -alpha with hKid triggers the initial targeting of hKid to mitotic chromosomes and that local Ran-GTP-mediated cargo release promotes the accumulation of hKid on chromosomes. Thus, this study demonstrates a novel nucleocytoplasmic transport factor-mediated mechanism for targeting proteins to mitotic chromosomes. |
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