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JXB Advance Access published online on November 16, 2005
Journal of Experimental Botany, doi:10.1093/jxb/erj010
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© The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received July 15, 2005
Accepted October 3, 2005

RESEARCH PAPER

CK2 phosphorylation weakens 90 kDa MFP1 association to the nuclear matrix in Allium cepa

Rafael Samaniego 1, Sun Yong Jeong 2, Consuelo de la Torre 3, Iris Meier 4, and Susana Moreno Díaz de la Espina 5 *

1 Nuclear Matrix Laboratory, Centro de Investigaciones Biológicas, CSIC, 28040-Madrid, Spain; Present address: Confocal Facility and Immuno-oncology Laboratory, Gregorio Marañón Hospital, Madrid, Spain
2 Plant Biotechnology Center and Department of Plant Biology, Ohio State University, Columbus, OH 43210, USA; Present address: Department of Biology, Duke University, Durham, NC 27708, USA
3 Cell Reproduction Laboratory, Centro de Investigaciones Biológicas, CSIC, 28040-Madrid, Spain
4 Plant Biotechnology Center and Department of Plant Biology, Ohio State University, Columbus, OH 43210, USA
5 Nuclear Matrix Laboratory, Centro de Investigaciones Biológicas, CSIC, 28040-Madrid, Spain

* To whom correspondence should be addressed.
Susana Moreno Díaz de la Espina, E-mail: smoreno{at}cib.csic.es


  Abstract

MFP1 is a conserved plant coiled-coil protein located on the stroma side of the chloroplast thylakoids, as well as in the nuclear matrix. It displays species-specific variability in the number of genes, proteins, and expression. Allium cepa has two nuclear proteins antigenically related to MFP1 with different Mr, pI, distribution, and expression, but only the 90 kDa MFP1 protein is a nuclear matrix component that associates with both the nucleoskeletal filaments and a new category of nuclear bodies. The 90 kDa AcMFP1 migrates in two-dimensional blots as two sets of spots. The hypo-phosphorylated forms (pI ~9.5) are tightly bound to the nuclear matrix, while high ionic strength buffers release the more acidic hyper-phosphorylated ones (pI ~8.5), suggesting that the protein is post-translationally modified, and that these modifications control its attachment to the nuclear matrix. Dephosphorylation by exogenous alkaline phosphatase and phosphorylation by exogenous CK2, as well as specific inhibition and stimulation of endogenous CK2 with heparin and spermine and spermidine, respectively, revealed that the protein is an in vitro and in vivo substrate of this enzyme, and that CK2 phosphorylation weakens the strength of its binding to the nuclear matrix. In synchronized cells, the nuclear 90 kDa AcMFP1 phosphorylation levels vary during the cell cycle with a moderate peak in G2. These results provide the first evidence for AcMFP1 in vivo phosphorylation, and open up further research on its nuclear functions.

Keywords: Allium cepa; cell cycle; CK2 phosphorylation; MFP1; nuclear matrix.
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