Abstract
BackgroundNucleoporin Nup88, a component of nuclear pore complexes, is known to be overexpressed in several types of tumor tissue. The overexpression of Nup88 has been reported to promote the early step of tumorigenesis by inducing multinuclei in both HeLa cells and a mouse model. However, the molecular basis of how Nup88 leads to a multinucleated phenotype remains unclear because of a lack of information concerning its binding partners. In this study, we characterize a novel interaction between Nup88 and vimentin. We also examine the involvement of vimentin in the Nup88-dependent multinucleated phenotype.MethodsCells overexpressing tagged versions of Nup88, vimentin and their truncations were used in this study. Coprecipitation and GST-pulldown assays were carried out to analyze protein-protein interactions. Vimentin knockdown by siRNA was performed to examine the functional role of the Nup88-vimentin interaction in cells. The phosphorylation status of vimentin was analyzed by immunoblotting using an antibody specific for its phosphorylation site.ResultsVimentin was identified as a Nup88 interacting partner, although it did not bind to other nucleoporins, such as Nup50, Nup214, and Nup358, in HeLa cell lysates. The N-terminal 541 amino acid residues of Nup88 was found to be responsible for its interaction with vimentin. Recombinant GST-tagged Nup88 bound to recombinant vimentin in a GST-pulldown assay. Although overexpression of Nup88 in HeLa cells was observed mainly at the nuclear rim and in the cytoplasm, colocalization with vimentin was only partially detected at or around the nuclear rim. Disruption of the Nup88-vimentin interaction by vimentin specific siRNA transfection suppressed the Nup88-dependent multinucleated phenotype. An excess amount of Nup88 in cell lysates inhibited the dephosphorylation of a serine residue (Ser83) within the vimentin N-terminal region even in the absence and presence of an exogenous phosphatase. The N-terminal 96 amino acid residues of vimentin interacted with both full-length and the N-terminal 541 residues of Nup88.ConclusionsNup88 can affect the phosphorylation status of vimentin, which may contribute to the Nup88-dependent multinucleated phenotype through changing the organization of vimentin.
Highlights
Nucleoporin Nup88, a component of nuclear pore complexes, is known to be overexpressed in several types of tumor tissue
Nup88 interacts with vimentin intermediate filament protein We attempted to establish stable cell lines expressing either Green fluorescent protein (GFP) or GFP-tagged Nup88 (Nup88-GFP) as materials for screening Nup88 interacting proteins and for subsequent experiments
Because overexpression of Nup88 in HeLa cells is reported to induce multipolar spindles and the appearance of multinucleated cells [16], a tet-on expression system was applied in order to reduce both cell death and unexpected defects caused by Nup88 overexpression during the development of the stable cell lines
Summary
Nucleoporin Nup, a component of nuclear pore complexes, is known to be overexpressed in several types of tumor tissue. Nuclear pore complexes (NPCs) are channels that mediate bidirectional trafficking between the cytoplasm and the nucleus in eukaryotic cells. A single NPC exhibits an eight-fold symmetric cylinder-like structure that can be divided into three distinct regions [2, 3]. One such region is the central channel, which spans the inner and outer nuclear membrane. FG repeat-containing Nups form unstructured barriers in the central channel, which are crucial for the selective trafficking of macromolecules [4, 5]
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