Iris Meier

Department of Plant Biology, The Ohio State University, Columbus OH 43201

First described in 1831, the cell nucleus is one of the best known but least understood of cellular organelles. Understanding in molecular detail the organizing principles of the nucleus - including the organization of chromatin and the coordination and regulation of synthesis, processing, and transport of macromolecules - is a major goal for cell biology. Recently, it has been suggested that nuclear processes such as replication, transcription, and splicing are spatially organized and associated with a nuclear framework called the nuclear matrix, a structure of unknown molecular composition. It has been shown that chromatin is attached to the nuclear matrix via specific DNA fragments called matrix attachment regions (MARs).

We have begun to dissect the plant nuclear matrix by isolating a DNA binding protein with specific affinity for MARs [1]. MAR binding filament-like protein 1 (MFP1) is associated with speckle-like structures at the nuclear rim that are a component of isolated nuclei and of the nuclear matrix. A predicted N-terminal transmembrane domain is necessary for the specific targeting of MFP1 to the speckles, indicating an association with the nuclear envelope [2]. We have identified a novel protein that specifically interacts with MFP1 in yeast two-hybrid and in vitro binding assays. MFP1 associated factor 1 (MAF1) is a small, soluble, serine/threonine-rich protein that is ubiquitously expressed and has no similarity to known proteins. MAF1, like MFP1, is located at the nuclear periphery and is a component of the nuclear matrix [3]. Together, these data indicate that MFP1 and MAF1 are components of a nuclear substructure, previously undescribed in plants, that connects the nuclear envelope and the internal nuclear matrix, and suggest a function for MFP1 in attaching chromatin to specific sites at the nuclear periphery.

We have identified the genes coding for MFP1 and MAF1 in the model plant Arabidopsis thaliana and are now initiating a reverse genetic approach to identify knock-out mutations in the two genes. These mutants will be valuable tools to understand the in vivo function of the novel proteins. We will search for phenotypic alterations on a morphological, cell-biological and molecular level. Combining the use of Green Fluorescent Protein (GFP) fusion protein expression in live plant cells with high-resolution confocal microscopy, we will follow the localization pattern of MFP1 and MAF1 during cell division, when the chromatin condenses and the nuclear envelope dissolves. The study of the localization of similar proteins from animals has provided some of the first molecular insights into the complex process of disintegration and re-establishment of the nucleus during cell division. Any information about similar molecular events in the even less characterized plant nucleus would be very valuable.

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