Mitosis-Specific Regulation of Nuclear Transport by the Spindle Assembly Checkpoint Protein Mad1p

Abstract

Nuclear pore complexes (NPCs) and kinetochores perform distinct tasks, yet their shared ability to bind several proteins suggests their functions are intertwined. Among these shared proteins is Mad1p, a component of the yeast spindle assembly checkpoint (SAC). Here we describe a role for Mad1p in regulating nuclear import that employs its ability to sense a disruption of kinetochore-microtubule interactions during mitosis. We show that kinetochore-microtubule detachment arrests nuclear import mediated by the transport factor Kap121p through a mechanism that requires Mad1p cycling between unattached, metaphase kinetochores and binding sites at the NPC. This signaling pathway requires the Aurora B-like kinase Ipl1p, and the resulting transport changes inhibit the nuclear import of Glc7p, a phosphatase that acts as an Ipl1p antagonist. We propose that a distinct branch of the SAC exists in which Mad1p senses unattached kinetochores and, by altering NPC transport activity, regulates the nuclear environment of the spindle.

Mad1 kinetochore recruitment by Mps1-mediated phosphorylation of Bub1 signals the spindle checkpoint

“…6B). However, Bub1-associated Mad1 is phosphorylated in vivo, suggesting that Mad1 phosphorylation may mediate other functions of Mad1 (Cairo et al 2013). The interaction between Bub1 and Mad1 also requires Mad2, as was previously reported in budding yeast in vivo (Brady and Hardwick 2000;Gillett et al 2004).…”

“…Microtubule binding could dissociate Mad1 from the kinetochore through a variety of mechanisms, such as active stripping of Mad1 and/or activation of a protein phosphatase. However, Mps1 overexpression results in Mad1 kinetochore localization even in the presence of normal kinetochoremicrotubule attachments (Hardwick et al 1996;Jelluma et al 2010;Cairo et al 2013), suggesting that Mps1 activity toward Bub1 must also be silenced to release Mad1. Activity of Mps1 toward Bub1 could be altered by spatial separation that occurs upon tension-induced stretching or conformational changes within Bub1/3 or Mps1 upon microtubule attachment.…”

The Composition, Functions, and Regulation of the Budding Yeast Kinetochore

“…Mad1 and mad2 mutants also exhibit segregation defects and genetic interactions that are stronger than a mad3 deletion, suggesting they also have separable segregation functions (Warren et al 2002;Daniel et al 2006). Mad1 was recently implicated in regulating nuclear transport, which may indirectly affect segregation because the import of the PP1/Glc7 phosphatase, discussed below, is affected (Cairo et al 2013). However, this role is independent of Mad2 so there are likely additional functions for Mad1 and Mad2 in chromosome segregation.…”

The spindle assembly checkpoint promotes chromosome bi-orientation: A novel Mad1 role in chromosome alignment

“…49,50 Several functions are reported for Mad1 localizing at the nuclear pore: pre-mitotic anaphase inhibitor formation, 49 Mad1-Mad2 proteostasis, 51 and the regulation of nuclear-cytoplasmic transport. 52 Most recently, the kinesin-binding motif was mapped at the very end of the amino-terminus, which associates directly with Cut7/kinesin-5 in fission yeast and with CENP-E/kinesin-7 in human cells 21 (Fig. 4).…”