Characterisation and high-resolution distribution of a matrix attachment region-binding protein (MFP1) in proliferating cells of onion

The first matrix attachment region (MAR)-binding protein sequenced in plant s, MFP1, has been characterised in two dicot species. Based on their antige nic relationship, we report here the conservation of MFP1-like proteins in proliferating root cells of onion (Allium cepa L). Two MFP1-like proteins w ith different molecular masses and solubilities were detected. The most abu ndant was a 90-kDa basic protein, presenting several separate spots in two- dimensional blots. The MFP1 was partially soluble and, similar to the proli ferating cell nuclear antigen (PCNA)-labelled replication factories in the nucleus and nuclear matrix, was localised at discrete foci as detected by c onfocal microscopy. High-resolution immunolocalisation of MFP1 by electron microscopy identified the foci as nuclear structures, some of them containi ng PCNA, which are ultrastructurally similar to the replication factories d escribed in animal cells. Our data provide the first report on MFP1-like pr oteins in the Alliaceae. In addition, we present evidence of the presence o f AcMFP1 in the putative replication factories.