PLANT NUCLEAR MATRIX/NUCLEAR ENVELOPE PROTEINS: PLANT-SPECIFIC COMPONENTS OF CHROMATIN ORGANIZATION?

Iris Meier1, Frank Gindullis2, and Nancy J. Peffer2

1Dept. of Plant Biol., Ohio State University, Columbus, OH 43220; and 2Dupont CR&D, Wilmington, DE 19880.

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. We have shown that MAR binding filament-like protein 1 (MFP1) is associated with speckle-like structures at the nuclear rim that are part of isolated nuclei and 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-endoplasmatic reticulum continuum. 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 know proteins. MAF1, like MFP1, is located at the nuclear rim and is a component of the nuclear matrix. 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. In animal cells, the nuclear lamins, intermediate filament proteins that form a layer beneath the nuclear envelope, are involved in organizing chromatin at the nuclear rim. Since neither the fully sequenced yeast genome, nor the partially completed Arabidopsis genome contain nuclear lamin genes, it is conceivable that alternative strategies to attach chromatin to the nuclear envelope have evolved in the non-animal kingdoms, and that MFP1 and MAF1 are involved in such organization in plants.