NUCLEAR DISTRIBUTION OF MFP1-LIKE PROTEINS IN RELATION TO CELL ACTIVITY IN ONION
Samaniego, R and Moreno Díaz de la Espina, S
Nuclear Matrix Laboratory, CIB CSIC, Madrid, Spain MFP1 is a protein of the plant nuclear matrix with both a long coiled-coil domain and a C-terminal DNA binding sub-domain. Their orthologs have been sequenced in Arabidopsis thaliana, tomato, tobacco and rice, but its functionality is currently unknown. We have used two domain-specific sera against LeMFP1 (288) and AtMFP1 (OSU 91) to look for proteins sharing antigenic determinants in onion. Both sera recognised a 78 kD protein, that is removed from nuclei after DNA digestion and high salt extraction. A second protein 90 kD and basic pI (8.5-9.4) was exclusively detected by serum 288, and presents different phosphorylation states that determine its binding to the NM. Both proteins are ubiquitous nuclear components in meristematic (either proliferative or quiescent) and differentiated cells, and show a dynamic pattern of nuclear distribution. The 78 kD protein distributes in small foci displaying a punctated nuclear pattern, that re-distributes during S-phase in hydroxyurea synchronised cells, and appear conserved in differentiated cells of roots and leaves. It localizes to nucleoli only in dormant and differentiated cells. The 90 kD protein accumulates in large nuclear foci associated to the NM, which are more abundant in proliferating cells. The small and large foci observed by confocal microscopy are the counterpart of the clusters of gold particles at the chromatin peripheries, and of dense nuclear bodies, respectively. These bodies are different from CBs and other nuclear bodies so far described in plants. On view of these data and also of previous results from our group and others, the relationships of the onion proteins with the sequenced homologues of tomato and Arabidopsis, and also the functionality of these proteins in the plant nucleus are discussed.
