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.