Two Distinct Repeat Sequences of Nup98 Nucleoporins Characterize Dual Nuclei in the Binucleated Ciliate Tetrahymena

Abstract

Ciliated protozoa have two functionally distinct nuclei, a micronucleus (MIC) and a macronucleus (MAC) [1]. These two nuclei are distinct in size, transcriptional activity, and division cycle control, proceeding with cycles of DNA replication and nuclear division at different times within the same cell [2, 3]. The structural basis generating functionally distinct nuclei remains unknown. Here, we show that, in Tetrahymena thermophila, the nuclear pore complexes (NPCs) of MIC and MAC are composed of different sets of nucleoporins. Among the 13 nucleoporins identified, Nup98 homologs were of interest because two out of the four homologs were localized exclusively in the MAC and the other two were localized exclusively in the MIC. The two MAC-localizing Nup98s contain repeats of GLFG [4]. In contrast, the two MIC-localizing Nup98s lack the GLFG repeats and instead contain a novel repeat signature of NIFN. Ectopic expression of a chimeric MIC-localizing Nup98 homolog bearing GLFG repeats obstructed the nuclear accumulation of MIC-specific nuclear proteins, and expression of a chimeric MAC-localizing Nup98 homolog bearing NIFN repeats obstructed the nuclear accumulation of MAC-specific nuclear proteins. These results suggest that Nup98s act as a barrier to misdirected localization of nucleus-specific proteins. Our findings provide the first evidence that the NPC contributes to nucleus-selective transport in ciliates.

Flexible Gates: Dynamic Topologies and Functions for FG Nucleoporins in Nucleocytoplasmic Transport

“…This was sufficient to drive import of micronucleus-specific factors into the macronucleus and vice versa. Thus, the FG-domain composition of NPCs differentially directs macronuclear versus micronuclear import in Tetrahymena (86,109). These in vivo studies indicate that there are complex requirements for combinations of FG domains for transport via different transport receptors and that the FG repeat composition of an NPC dictates its transport permissibility.…”

“…The FG-Nups of higher eukaryotes also arrange in distinct substructural locations within the pore (159). FG-Nups are anchored into these specific NPC locations and Nup subcomplexes by their non-FG domains, and deletion of these non-FG domains results in mistargeting, (8,39,45,52,80,86,171,181). Coiled-coil motifs in the non-FG domains of FG-Nups are predicted to provide this NPC anchoring function (44).…”

Heterochromatin aggregation during DNA elimination in Tetrahymena is facilitated by a prion-like protein

“…S1A). One of these proteins was the MIC-specific nuclear pore protein MicNup98B (Iwamoto et al, 2009;Malone et al, 2008), which is unlikely to be involved in heterochromatin body formation in the new MAC. Therefore, we further analyzed the other four proteins: TTHERM_01415190, TTHERM_00647510, TTHERM_00145310 and TTHERM_ 00421130 (Tetrahymena Genome Database).…”

Nuclear size is sensitive to NTF2 protein levels in a manner dependent on Ran binding

“…Blocking nuclear export increases nuclear size in fission yeast (Neumann and Nurse, 2007), and altering nucleoporin (Nup) expression affects nuclear size in Tetrahymena, Arabidopsis, and Xenopus (Iwamoto et al, 2009;Tamura et al, 2010;Theerthagiri et al, 2010;Shaulov et al, 2011;Tamura and Hara-Nishimura, 2011). In addition, manipulating the expression of various nuclear transport factors causes nuclear size changes in Tetrahymena, worms and budding yeast (Malone et al, 2008;Gonzalez et al, 2009;Meyerzon et al, 2009).…”