K. Suzuki et al., CYTOKINESIS BY A CONTRACTILE RING IN THE PRIMITIVE RED ALGA CYANIDIUM-CALDARIUM RK-1, European journal of cell biology, 67(2), 1995, pp. 170-178
To better understand the mechanism of cytokinesis is eukaryotes, the b
ehavior of the contractile ring in the two unicellular primitive red a
lgae Cyanidioschyzon merolae and Cyanidium caldarium RK-1, which have
the smallest genome size among eukaryotes, was examined by fluorescein
isothiocyanate (FITC)-phalloidin fluorescence microscopy, fluorometry
using a video-intensified microscope photon-counting system, transmis
sion electron microscopy and immunoblotting techniques. Cells in each
alga contained one nucleus, one mitochondrion and one chloroplast, whi
ch were aligned in that order. During cytokinesis in C. merolae, a con
tractile ring was not observed by fluorescence microscopy or by transm
ission electron microscopy. In contrast, in C. caldarium RK-1, a contr
actile ring appeared at the equatorial region of the dividing cells an
d began to contract from the side of the chloroplast. During contracti
on of this ring, the total fluorescent intensifies due to FITC-phalloi
din remained constant. Electron microscopy revealed outer and inner ba
nds approximately 80 mm wide and 9 nm thick which ran parallel to each
other just beneath the cell membrane. These bands were visible at the
equator of the cell just before the initiation of cytokinesis and con
stricted from the pole of the chloroplast. Both bands increased in wid
th as cleavage progressed. The inner ring consisted of a bundle of app
roximately 20 actin-like filaments which were arranged as a raft. In t
he outer ring, such fine filaments were not visible. It seems likely t
hat the bundle of filaments, known as the contractile ring, is compose
d of two different elements: an inner band of actin filaments and an o
uter band of unknown materials. Western immunoblotting with anti-actin
antibody gave negative results in C. merolae, but was positive in C.
caldarium RK-1. These results indicate that the contractile ring in C.
caldarium RK-I consists of a constant amount of actin filaments throu
ghout cytokinesis and that cleavage is caused by the contractile ring.
Furthermore, these results suggest that C. merolae may use markedly d
ifferent actin than C. caldarium RK-1 in contractile mechanisms.