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Vol. 16, Issue 9, 4362-4374, September 2005

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Interactions between Mad1p and the Nuclear Transport Machinery in the Yeast Saccharomyces cerevisiae

Robert J. Scott ^*, C. Patrick Lusk ^*, David J. Dilworth , John D. Aitchison , and Richard W. Wozniak ^*

^* Department of Cell Biology, University of Alberta, Edmonton, Alberta, Canada, T6G 2H7; Institute for Systems Biology, Seattle, WA 98103

Submitted January 7, 2005; Revised June 28, 2005; Accepted June 29, 2005
Monitoring Editor: Susan Wente

In addition to its role in nucleocytoplasmic transport, the nuclear pore complex (NPC) acts as a docking site for proteins whose apparent primary cellular functions are unrelated to nuclear transport, including Mad1p and Mad2p, two proteins of the spindle assembly checkpoint (SAC) machinery. To understand this relationship, we have mapped domains of yeast Saccharomyces cerevisiae Mad1p that interact with the nuclear transport machinery, including further defining its interactions with the NPC. We showed that a Kap95p/Kap60p-dependent nuclear localization signal, positioned in the C-terminal third of Mad1p, is required for its efficient targeting to the NPC. At the NPC, Mad1p interacts with Nup53p and a presumed Nup60p/Mlp1p/Mlp2p complex through two coiled coil regions within its N terminus. When the SAC is activated, a portion of Mad1p is recruited to kinetochores through an interaction that is mediated by the C-terminal region of Mad1p and requires energy. We showed using photobleaching analysis that in nocodazole-arrested cells Mad1p rapidly cycles between the Mlp proteins and kinetochores. Our further analysis also showed that only the C terminus of Mad1p is required for SAC function and that the NPC, through Nup53p, may act to regulate the duration of the SAC response.

Abbreviations used: NE, nuclear envelope; NPC, nuclear pore complex; SAC, spindle assembly checkpoint.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Richard W. Wozniak (rick.wozniak{at}ualberta.ca).

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