A POSSIBLE ROLE FOR ACTIN DOTS IN THE FORMATION OF THE CONTRACTILE RING IN THE ULTRA-MICRO ALGA CYANIDIUM-CALDARIUM RK-1

In the primitive red alga Cyanidium caldarium RK-1, cytokinesis is con trolled by a simple contractile ring, as in animal cells. To clarify t he mechanism of formation of the contractile ring, we isolated actin g enes and performed an immunocytological study. C. caldarium RK-I has t wo actin genes encoding proteins with the same sequence of 377 amino a cids. The primary structure indicated that the actin molecules of C. c aldarium RK-I are typical, despite the fact that the organism is consi dered to be phylogenetically primitive. We prepared antiserum against a C. caldarium RK-1 actin fusion protein and indirect immunofluorescen ce staining was performed. In interphase cells, many actin dots were o bserved in the cytoplasm but none at the future cleavage plane. Prior to cytokinesis, some of these dots appeared and became aligned along t he equatorial plane. At the same time, a thin ''immature'' contractile ring was observed to appear to be formed by connection of the aligned actin dots. This immature contractile ring thickened to nearly its ma ximum size by the time cytokinesis began. The formation of the contrac tile ring seemed to be a result of de novo assembly of actin monomers, rather than a result of the accumulation and bundling of pre-existing actin filaments. During the constriction of the contractile ring, no actin dots were observed in the cytoplasm. These observations suggest that actin dots are responsible for the formation of the contractile r ing, but are not necessary for its disintegration. Furthermore, immuno gold localization specific for actin revealed at electron microscopy l evel that fine filaments running just beneath the cleavage furrow are, in fact, actin filaments.