THE PLANT NUCLEOLIN-LIKE PROTEIN NopA100 IS PROTEOLYZED AND RE-ORGANIZED IN RELATION TO THE NUCLEOLAR ACTIVITY AND CELL CYCLE PROGRESSION
González-Camacho F., Medina F.J.
Centro de Investigaciones Biológicas (CSIC), Madrid, Spain. The nucleolar protein NopA100 has been detected in the soluble nuclear fraction from onion proliferating cells. From its physico-chemical properties, and from its immunological cross-reactions, it can be identified as a nucleolin-like protein. In a cell population of root meristems synchronized with hydroxyurea the levels of the protein were shown to increase in the G2 phase of cell cycle. Light and electron microscopical immuno-cytochemistry revealed an unequal distribution within the nucleolus appearing mainly in the dense fibrillar component, near fibrillar centers. A Western blot analysis with anti-NopA100 antibody resulted in the intense labeling of a band of 100 kDa, but also of a series of proteins related with it, one of them higher in molecular mass (120 kDa) and several of them below this level, suggesting that NopA100 undergoes a physiological process of proteolytic maturation, similar to that described for mammalian nucleolin, but not reported for other nucleolin-like proteins from Xenopus, other plants and yeasts. There is a relationship between the increase of the activity of nucleolus, the activity of NopA100, the progress of the cell cycle and NopA100 proteolysis. This relationship has been evidenced after a treatment with leupeptin, a general inhibitor of proteases, of nuclei extracted from synchronized cells, which resulted in the differential quantitative diminution of the bands product of the proteolysis at the expenses of an increase of the 100 kDa main band, according to cell cycle stages. The variations described in different features of NopA100 during the cell cycle are accompanied by a structural rearrangement of the nucleolus, which is expressed in quantitative changes of the relative contribution of the different nucleolar subcomponents to the nucleolar volume. A structural model of nucleolar organization can be designed for each one of the periods of the interphase in onion proliferating cells.
