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Interphase-specific phosphorylation-mediated regulation of tubulin dimer partitioning in human cells.
Umeå University, Faculty of Medicine, Molecular Biology (Faculty of Medicine). (Gullberg)
Umeå University, Faculty of Medicine, Molecular Biology (Faculty of Medicine). (Gullberg)
Umeå University, Faculty of Medicine, Molecular Biology (Faculty of Medicine). (Gullberg)
2007 (English)In: Molecular biology of the cell, ISSN 1059-1524, Vol. 18, no 5, 1909-1917 p.Article in journal (Refereed) Published
Abstract [en]

The microtubule cytoskeleton is differentially regulated by a diverse array of proteins during interphase and mitosis. Op18/stathmin (Op18) and microtubule-associated protein (MAP)4 have been ascribed opposite general microtubule-directed activities, namely, microtubule destabilization and stabilization, respectively, both of which can be inhibited by phosphorylation. Here, using three human cell models, we depleted cells of Op18 and/or MAP4 by expression of interfering hairpin RNAs and we analyzed the resulting phenotypes. We found that the endogenous levels of Op18 and MAP4 have opposite and counteractive activities that largely govern the partitioning of tubulin dimers in the microtubule array at interphase. Op18 and MAP4 were also found to be the downstream targets of Ca(2+)- and calmodulin-dependent protein kinase IV and PAR-1/MARK2 kinase, respectively, that control the demonstrated counteractive phosphorylation-mediated regulation of tubulin dimer partitioning. Furthermore, to address mechanisms regulating microtubule polymerization in response to cell signals, we developed a system for inducible gene product replacement. This approach revealed that site-specific phosphorylation of Op18 is both necessary and sufficient for polymerization of microtubules in response to the multifaceted signaling event of stimulation of the T cell antigen receptor complex, which activates several signal transduction pathways.

Place, publisher, year, edition, pages
2007. Vol. 18, no 5, 1909-1917 p.
Keyword [en]
Base Sequence, Ca(2+)-Calmodulin Dependent Protein Kinase/metabolism, Cell Line, DNA/genetics, Dimerization, Humans, Interphase/*physiology, Jurkat Cells, K562 Cells, Microtubule-Associated Proteins/antagonists & inhibitors/genetics/metabolism, Microtubules/metabolism, Phosphorylation, Protein Structure; Quaternary, Protein-Serine-Threonine Kinases/metabolism, Signal Transduction, Stathmin/antagonists & inhibitors/genetics/metabolism, Transfection, Tubulin/*chemistry/*metabolism
URN: urn:nbn:se:umu:diva-16490DOI: 10.1091/mbc.E07-01-0019PubMedID: 17344472OAI: diva2:156163
Available from: 2007-10-03 Created: 2007-10-03Bibliographically approved
In thesis
1. Regulation of tubulin heterodimer partitioning during interphase and mitosis
Open this publication in new window or tab >>Regulation of tubulin heterodimer partitioning during interphase and mitosis
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The microtubule cytoskeleton, which consists of dynamic polymers of alpha/beta tubulin heterodimers, organizes the cytoplasm and is essential for chromosome segregation during mitosis. My thesis addresses the significance and potential interplay between four distinct microtubule-regulatory proteins. The experimental approach included the development of a replicating vector system directing either constitutive expression of short hairpin RNAs or inducible ectopic expression, which allows stable depletion and/or conditional exchange of gene-products.

Based on the originally observed activities in frog egg extracts, MCAK and TOGp have been viewed as major antagonistic proteins that regulate microtubule-dynamics throughout the cell cycle. Surprisingly, while my thesis work confirmed an essential role of these proteins to ensure mitotic fidelity, tubulin subunits partitioning is not controlled by the endogenous levels of MCAK and TOGp in human somatic cells. Our major discovery in these studies is that the activities of both CaMKII and TOGp are essential for spindle bipolarity through a mechanism involving protection of spindle microtubules against MCAK activity at the centrosome.

In our search for the major antagonistic activities that regulates microtubule-dynamics in interphase cells, we found that the microtubule-destabilizing activity of Op18 is counteracted by MAP4. These studies also established Op18 and MAP4 as the predominant regulators of tubulin subunit partitioning in all three human cell model systems studied. Moreover, consistent with phosphorylation-inactivation of these two proteins during mitosis, we found that the microtubule-regulatory activities of both MAP4 and Op18 were only evident in interphase cells. Importantly, by employing a system for inducible gene product replacement, we found that site-specific phosphorylation-inactivation of Op18 is the direct cause of the demonstrated hyper-polymerization in response to T-cell antigen receptor triggering. This provides the first formally proven example of a signal transduction pathway for regulation of interphase microtubules.

Op18 is frequently upregulated in various types of human malignancies. In addition, a somatic mutation of Op18 has recently been identified in an adenocarcinoma. This thesis work revealed that the mutant Op18 protein exerts increased microtubule-destabilizing activity. The mutant Op18 protein was also shown to be partially resistant to phosphorylation-inactivation during mitosis, which was associated with increased chromosome segregation aberrancies. Interestingly, we also observed the same phenotype by overexpressing the wild type Op18 protein. Thus, either excessive levels of wild type Op18 or normal levels of mutated hyper-active Op18 seems likely to contribute to tumor progression by exacerbating chromosomal instability.

Place, publisher, year, edition, pages
Umeå: Molekylärbiologi (Medicinska fakulteten), 2008. 30 p.
Umeå University medical dissertations, ISSN 0346-6612 ; 1220
microtubule, mitotic spindle, signal transduction, phosphorylation, cancer
National Category
Biochemistry and Molecular Biology
urn:nbn:se:umu:diva-1923 (URN)978-91-7264-670-4 (ISBN)
Public defence
2008-12-16, Major groove, 6L, Umeå, 10:00 (English)
Available from: 2008-11-18 Created: 2008-11-18 Last updated: 2010-01-18Bibliographically approved

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Holmfeldt, PerStenmark, SonjaGullberg, Martin
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