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F-actin ring formation and the role of F-actin cables in the fission yeast Schizosaccharomyces pombe

Ritsuko Arai1 and Issei Mabuchi1,2,*

1 Division of Biology, Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, 153-8902, Japan
2 Department of Cell Biology, National Institute for Basic Biology, Okazaki, 444-8585, Japan



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  		Fig. 6. F-actin structures in nda3 mutant cells. nda3 mutant cells were incubated at 20°C for 8 hours to arrest. For release experiment, once arrested, cells were re-incubated at 36°C and collected every 2 minutes. These cells were immediately fixed at each time, and then stained simultaneously with DAPI and Bodipyphallacidin. (a-h) Photographs obtained by conventional fluorescence microscopy. Left, DAPI stainings; right, Bodipy-phallacidin stainings. (a,b) Arrested cells. (c-h) Released cells: 4 minutes (c,d), 10 minutes (e,f) and 14 minutes (g,h) after temperature shift-up. (i,j) 3D images. Green and blue represent F-actin and DNA, respectively. (i) Arrested cells. (j) Cells at 4 minutes after the release. Arrows indicate accumulated F-actin cables. Bar, 2 µm.

 


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  		Fig. 1. F-actin structures in a wild-type cell during interphase. Green, red and blue represent F-actin, SPBs and DNA, respectively. A representative 3D image is shown in a. (b) Illustration of the F-actin cables in the cell. Thick lines represent the F-actin cables at one side of the cell, whereas thin lines represent those at the other side of the cell. Deconvoluted serial sections are shown in c-n. A magnified image of SPB is shown in e. Three major F-actin cables are seen in this cell (white arrows). Some of the F-actin patches are linked to the F-actin cables (pink arrows). Bars, 2 µm (a,n); 1 µm (e).

 


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  		Fig. 2. F-actin structures in three wild-type cells at early prophase. Green, red and blue (a-f) represent F-actin, SPBs and DNA, respectively. A representative 3D image of one of these cells is shown in a. Deconvoluted serial sections around the SPBs of these cells are shown in b-f, g-j and k-o, respectively. The SPBs just initiate to separate (pink arrows; see magnified images in insets of d, i and k, respectively), and some F-actin cables (white arrows) are branched near the SPBs at the medial region of the cell. Arrowheads indicate F-actin patches attached to F-actin cables. Bars, 2 µm (o); 1 µm (k).

 


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  		Fig. 3. F-actin structures in wild-type cells at metaphase. Green, red and blue represent F-actin, SPBs and DNA, respectively. (A) A cell in which a single leading cable extends from the aster-like structure. (B) A cell in which two leading cables extend from the aster-like structure. Rotating 3D images are shown in a and b (0° and 12°, respectively) of each panel. Direction of rotation is marked near a. (c) The F-actin cables in each cell. Thick lines represent the F-actin cables, including the aster at one side of the cell, whereas thin lines represent those at the reverse side of the cell. Deconvoluted serial sections are shown in d-o (A) or d-s (B). Pink arrows indicate an aster-like structure, and white arrows indicate leading F-actin cables. White arrowheads indicate longitudinal F-actin cable. Bars, 2 µm.

 


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  		Fig. 4. F-actin structures in a wild-type cell at early anaphase. Green, red and blue represent F-actin, SPBs and DNA, respectively. Rotating 3D images are shown in a and b (0° and 24°, respectively). Direction of rotation is marked on the left of a. Deconvoluted serial sections are shown in c-m. F-actin ring is formed but partially distorted (pink arrow). Some F-actin cables are linked to the ring (white arrows). Bar, 2 µm.

 


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  		Fig. 5. F-actin structures in wild-type cells during anaphase and cytokinesis. Green, red and blue represent F-actin, SPBs and DNA, respectively. 3D images of mid and late anaphase cells (a and b, respectively), early and late cytokinesis cells (c and d, respectively) and separating cells (e) are shown. White arrows, F-actin cables. Pink arrows, F-actin patches. Bar, 2 µm.

 


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  		Fig. 7. F-actin structures in cdc12-112 mutant cells. Green, red and blue represent F-actin, SPBs and DNA, respectively. (A) Metaphase cell. (a) 3D image of chromosomes and SPBs. (b) 3D image of F-actin structures. (c) F-actin cables in this cell. Thick lines represent the cables at one side of the cell, whereas thin lines represent those at the other side of the cell. (d-q) Deconvoluted serial sections around a medial region of this cell. (B) Anaphase cell. (a) 3D image of chromosomes and SPBs. (b) 3D image of F-actin structures. (c) F-actin cables in this cell. (d-q) Deconvoluted serial sections around a medial region of this cell. (C) A cell after anaphase. (a) 3D image of chromosomes and SPBs. (b) 3D image of F-actin structures. Pink and white arrowheads represent pairs of sister SPBs, respectively. Pink arrows, aster-like structures. White arrows, F-actin cables. Bars, 2 µm

 


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  		Fig. 8. F-actin structures in cdc15-140 mutant cells. Green, red and blue represent F-actin, SPBs and DNA, respectively. (A) Metaphase cell. (a) 3D image of chromosomes and SPBs. (b,c) Rotating 3D images of F-actin structures (0° and 24°, respectively). Direction of rotation is marked on the right of c. (d-r) Deconvoluted serial sections around a medial region of this cell. Pink arrows, aster-like structures. White arrows, leading F-actin cables. (B) Anaphase cell. (a) 3D image of chromosomes and SPBs. Pink and white arrowheads represent pairs of sister SPBs, respectively. (b) 3D image of F-actin structures. (c-p) Deconvoluted serial sections around a medial region of this cell. Double F-actin ring-like structures (arrows) are formed. (C) A cell after anaphase. (a) 3D image of chromosomes and SPBs. Pink and white arrowheads represent pairs of sister SPBs, respectively. (b) 3D image of F-actin structures. (c-n) Deconvoluted serial sections around a medial region of this cell. A part of the F-actin ring still remains (white arrows). Some F-actin cables are elongated from a point (pink arrows). Bars, 2 µm.

 


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  		Fig. 9. F-actin structures in spg1-B8, cdc7-24 and sid2-250 mutant cells. Green, red and blue represent F-actin, SPBs and DNA, respectively. (a,b) A cdc7 mutant cell at late anaphase. (c,d) A sid2 mutant cell at late anaphase. An incomplete F-actin ring (pink arrow) is connected to a complete F-actin ring (white arrow). (e,f) A cdc7 mutant cell after anaphase. (g,h) spg1 mutant cells. White arrow, remaining F-actin ring. Pink arrow, a remnant of disappearing F-actin ring. (a,c,e,g) 3D images of chromosomes and SPBs. Pink and white arrowheads represent pairs of sister SPBs. (b,d,f,h) 3D images of F-actin structures. Bar, 2 µm.

 


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  		Fig. 10. A model for F-actin ring formation in the wild-type S. pombe cell. F-actin cables are elongated between both ends of the cell during interphase. As positional signal(s) is generated from unseparated SPBs and/or nucleus at pre-prophase, formation of an aster-like structure is initiated from a spot during prophase, mediated by Cdc12 or other unknown proteins. This process includes branching of the F-actin cable, which is closely associated with the SPBs. During metaphase, the aster formation and accumulation of the F-actin cables progress at the medial region. A leading F-actin cable extends from the aster and encircle the cytoplasm at the equator. Thus a primary F-actin ring is formed. Cdc12 may be involved in this primary ring formation. A thick F-actin ring is formed during anaphase by packing and fusion of the accumulated F-actin cables with the leading cable. Cdc15 may play a role in this process. During cytokinesis, the F-actin ring initiates contraction being regulated by the Spg1-Cdc7-Sid2 pathway, and then F-actin cables reappear.

 

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© The Company of Biologists Ltd 2002