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© The Rockefeller University Press, 0021-9525/1999//1173 $5.00
The Journal of Cell Biology, Volume 144, Number 6, , 1999 1173-1186


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Fission Yeast {alpha}-Glucan Synthase Mok1 Requires the Actin Cytoskeleton to Localize the Sites of Growth and Plays an Essential Role in Cell Morphogenesis Downstream of Protein Kinase C Function



Satoshi Katayama*, Dai Hirata*, Manuel Arellano{ddagger}, Pilar Pérez{ddagger}, and Takashi Toda*

* Laboratory of Cell Regulation, Imperial Cancer Research Fund, London WC2A 3PX, United Kingdom, {ddagger} Instituto de Microbiologia Bioquimica, CSIC/Universidad de Salamanca, Edificio Departamental, 37007 Salamanca, Spain

In fission yeast protein kinase C homologues (Pck1 and Pck2) are essential for cell morphogenesis. We have isolated mok1+ in a genetic screen to identify downstream effectors for Pck1/2. mok1+ is essential for viability and encodes a protein that has several membrane-spanning domains and regions homologous to glucan metabolic enzymes. mok1 mutant shows abnormal cell shape, randomization of F-actin and weak cell wall. Biochemical analysis shows that Mok1 appears to have {alpha}-glucan synthase activity. Mok1 localization undergoes dramatic alteration during the cell cycle. It localizes to the growing tips in interphase, the medial ring upon mitosis, a double ring before and dense dot during cytokinesis. Double immunofluorescence staining shows that Mok1 exists in close proximity to actin. The subcellular localization of Mok1 is dependent upon the integrity of the F-actin cytoskeleton. Conversely, overexpression of mok1+ blocks the translocation of cortical actin from one end of the cell to the other. pck2 mutant is synthetically lethal with mok1 mutant, delocalizes Mok1 and shows a lower level of {alpha}-glucan. These results indicate that Mok1 plays a crucial role in cell morphogenesis interdependently of the actin cytoskeleton and works as one of downstream effectors for Pck1/2.

Key Words: actin • {alpha}-glucan synthase • fission yeast • morphogenesis • protein kinase C



Abbreviations used in this paper: cs, cold-sensitive; GFP, green fluorescent protein; HA, hemagglutinin; lat A, latrunculin A; PKC, protein kinase C; STS, staurosporine; ts, temperature-sensitive.

Address correspondence to Takashi Toda, Laboratory of Cell Regulation, Imperial Cancer Research Fund, PO Box 123, 44 Lincoln's Inn Fields, London WC2A 3PX, UK. Tel.: 44 171 269 3535. Fax: 44-171-269-3258. E-mail: toda{at}europa.lif.icnet.uk

The initial part of this work was supported by a grant from Kyowa Hakko Co. Ltd.

Dr. Hirata's present address is Department of Molecular Biotechnology, Graduate School of Advanced Sciences of Matter, Hiroshima University, and "Unit Process and Combined Circuit," PRESTO, JST, Higashi-Hiroshima 739-8526, Japan.



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