CELL-CYCLE DEPENDENT DISTRIBUTION OF A CENTROSOMAL ANTIGEN AT THE PERINUCLEAR MTOC OR AT THE KINETOCHORES OF HIGHER-PLANT CELLS

Compelling evidence has been obtained in favour of the idea that the n uclear surface of higher plant cells is a microtubule-nucleating and/o r organizing site (MTOC), in the absence of defined centrosomes. How t hese plant MTOC proteins are redistributed and function during the pro gression of the cell cycle remains entirely unknown. Using a monoclona l antibody (mAb 6C6) raised against isolated calf thymus centrosomes a nd showing apparent reaction with the plant nuclear surface, we follow ed the targeted antigen distribution during mitosis and meiosis of hig her plants. Immunoblot analysis of protein fractions from Allium root meristematic cell extracts probed with mAb 6C6 reveals a polypeptide o f an apparent Mr of 78000. In calf centrosome extracts, a polypeptide of comparable molecular mass is found in addition to a major antigen o f Mr 180000 after mAb 6C6 immunoblotting. During mitotic initiation, t he plant antigen is prominent on the periphery of the prophase nucleus . When the nuclear envelope breaks down, the antigen suddenly becomes associated with the centromere-kinetochores until late anaphase. In te lophase, when the nuclear envelope is being reconstructed, it is no lo nger detected at the kinetochores but is solely associated again with the nuclear surface. This antigen displays a unique spatial and tempor al distribution, which may reflect the pathway of plant protein(s) bet ween the nuclear surface and the kinetochores under cell cycle control . So far, such processes have not been described in higher plant cells . These observations shed light on the putative activity of the plant kinetochore as a protein transporter. They also suggest that a plant c entrosome-like antigen may have different cytoskeletal related functio ns depending on cell cycle regulated changes in its subcellular distri bution.