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J. Biol. Chem., Vol. 281, Issue 15, 9935-9941, April 14, 2006

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Identification of Functional Domains within the Septation Initiation Network Kinase, Cdc7^*

Sapna Mehta and Kathleen L. Gould¹

From the Howard Hughes Medical Institute and Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232

The septation initiation network (SIN) serves to coordinate cytokinesis with mitotic exit in the fission yeast Schizosaccharomyces pombe. SIN components Spg1 and Cdc7 together play a central role in regulating the onset of septation and cytokinesis. Spg1, a Ras-like GTPase, localizes to the spindle pole bodies (SPBs) throughout the cell cycle. It is converted to its GTP-bound (active) state during mitosis, only to become inactivated at one SPB during anaphase and at both SPBs as cells exit mitosis. Cdc7 functions as an effector kinase for Spg1, binding to Spg1 in its GTP-bound state, and therefore is present at both SPBs during mitosis and asymmetrically at only one during anaphase. Interestingly, the kinase activity of Cdc7 does not vary across the cell cycle, suggesting the possibility that Cdc7 kinase activity is independent of Spg1 binding. Consistent with this, we found that Cdc7 associates with Spg1 only during mitosis. To learn more about the essential role of Cdc7 kinase in the SIN and its regulation, we undertook a structure/function analysis and identified independent functional domains within Cdc7. We found that a region adjacent to the kinase domain is responsible for Spg1 association and identified an overlapping but distinct SPB localization domain. In addition Cdc7 associates with itself and exists as a dimer in vivo.

Received for publication, April 11, 2005 , and in revised form, January 6, 2006.

^* This work was supported by the Howard Hughes Medical Institute. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Investigator of the Howard Hughes Medical Institute (HHMI). To whom correspondence should be addressed: HHMI and Dept. of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, TN 37232. Tel.: 615-343-9502; Fax: 615-343-0723; E-mail: kathy.gould{at}vanderbilt.edu.

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