Nucleocytoplasmic transport of Alp7/TACC organizes spatiotemporal microtubule formation in fission yeast

Abstract

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits distribution, and reproduction in any medium, provided the original author and source are credited. This license does not permit commercial exploitation without specific permission.Ran GTPase activates several target molecules to induce microtubule formation around the chromosomes and centrosomes. In fission yeast, in which the nuclear envelope does not break down during mitosis, Ran targets the centrosomal transforming acidic coiled-coil (TACC) protein Alp7 for spindle formation. Alp7 accumulates in the nucleus only during mitosis, although its underlying mechanism remains elusive. Here, we investigate the behaviour of Alp7 and its binding partner, Alp14/ TOG, throughout the cell cycle. Interestingly, Alp7 enters the nucleus during interphase but is subsequently exported to the cytoplasm by the Exportin-dependent nuclear export machinery. The continuous nuclear export of Alp7 during interphase is essential for maintaining the array-like cytoplasmic microtubule structure. The mitosis-specific nuclear accumulation of Alp7 seems to be under the control of cyclin-dependent kinase (CDK). These results indicate that the spatiotemporal regulation of microtubule formation is established by the Alp7/TACC-Alp14/ TOG complex through the coordinated interplay of Ran and CDK.

Space shuttling in the cell: Nucleocytoplasmic transport and microtubule organization during the cell cycle

“…It is experimentally difficult to discriminate these possibilities, but our results suggest the second possibility. 26 We have found that, during interphase, Alp7-Alp14 enters the nucleus in an NLSdependent manner and is exported to the cytoplasm in an NES-dependent manner. Upon entry into mitosis, the complex is retained in the nucleus to execute mitotic function, but NLS activity is not specifically potentiated.…”

“…2). [26][27][28] As discussed above, Alp7/TACCAlp14/TOG promotes assembly of the mitotic spindle. This complex is also essential to establish cytoplasmic microtubules in interphase, as shown by the alp14 mutant that exhibits fragile and fragmented cytoplasmic microtubules.…”

“…39 Thus, one might propose that Alp7/TACC is merely a nuclear import factor for Alp14, as the intrinsic NLS of Alp7 is responsible for the nuclear accumulation of Alp14/ TOG. 26 This does not appear to be the case, however, because fusion of the SV40-NLS to Alp14 (Alp14-NLS) does not suppress the mitotic defects of alp7∆ cells, although Alp14-NLS is capable of localizing to the nucleus independently of Alp7 (our unpublished results). This implies that Alp7 is more than just a nuclear import factor for Alp14.…”

“…When the expression of B-type cyclin Cdc13 is artificially shut off in meiosis, Alp7 does not accumulate in the nucleus. 26 One might argue that this is simply due to the failure to enter meiotic M phase in the absence of Cdc13, rather than support for the requirement of nucleocytoplasmic shuttling of this complex during interphase is crucial for interphase microtubule organization.…”

“…17 Indeed, artificial targeting of the Alp7-Alp14 complex to the nucleus in interphase leads to crumbling of cytoplasmic microtubules reminiscent of that observed in the alp14 mutant. 20,26 Thus, lacking the intrinsic NLSs would cause failure to assemble the heterodimer in the nucleus. Heterodimerization of Klp5 and Klp6 is essential to regulate microtubule dynamics; in each mutant the frequency of both catastrophe and rescue of microtubules is reduced, resulting in less dynamic microtubules than in wild type.…”