NUCLEAR AND CYTOPLASMIC ACTIN IN DINOFLAGELLATES

Experiments using monoclonal and polyclonal anti-actin antibodies allo wed us to demonstrate the presence of F- or G-actin in original protis ts, dinoflagellates, either by biochemistry, immunofluorescence and in TEM. SDS-PAGE electrophoresis and immunoblottings made either from to tal or nuclear protein extracts revealed the presence of a 44-kDa band reacting with monoclonal anti-actin antibody in two species, Prorocen trum micans and Crypthecodinium cohnii, and thus demonstrated the pres ence of actin in nuclear and cytoplasmic fractions. After squash prepa ration of P micans cells, actin was identified within the nucleus and in some regions of the cytoplasm by immunofluorescence microscopy. Lab elling of both the nucleolus and the centrosome region was evident tog ether with amorphous nucleoplasmic material surrounding the chromosome s. The use of cryosections of intact P micans and C coh nii cells for immunofluorescence along with staining with DAPI to delineate the chro mosomes themselves, yielded finer resolution of the intranuclear netwo rk labelling pattern and allowed us to complete our observations, in p articular on the cytoplasmic labelling. In P micans, in addition to th e centrosome region, the cytoplasmic channels passing through the nucl eus in dividing cells are labelled. In C cohnii, the cortex, the centr osome region, the cytoplasmic channels, the region surrounding the nuc leus, the filaments linking it to the cortex and the cleavage furrow a re also labelled. In the nucleus of the two species, there is a promin ent 'weft' of fine actin filaments in the nucleoplasm forming a matrix of varying density around the persistent chromosomes. This actin matr ix, of unknown function, is most conspicuous at the end of the S-phase of the cell cycle. Fluorescent derivatives of phalloidin, used as dia gnostic cytochemical probes for polymeric actin (F-actin), gave simila r results. Positive TEM immunolabelling of intranuclear actin confirms its presence in the nucleoplasm, in the nucleolus where the preriboso mal region is labelled while C cohnii chromosomes are unlabelled and t he P micans chromosomes very slightly. In the cytoplasm, lips of the c leavage furrow and kinetosome regions are labelled as well as the cent rosome region. The possible functions of this protein located in sever al compartments of dinoflagellate cells are discussed.