Figure 8.

 (A) Diagram showing the changes in localization of Cdc7p, Spg1p, and GDP–Spg1p during the cell cycle. Note that while Spg1p is always associated with the spindle pole body, it is predominantly in the GTP-bound form during mitosis. Cdc7p is only associated with the spindle pole body during mitosis and is segregated asymmetrically during anaphase B. (B) The signal for the onset of septum formation is delivered from the spindle pole body. The diagram summarizes our working hypothesis. Activation of a Cdc7p-dependent signaling pathway at the end of anaphase sends a signal from the spindle pole body to the medial ring to initiate synthesis of the division septum. Because the cell is primed for septation from the beginning of mitosis, the presence of a putative inhibitor during early mitosis is indicated. The nature of the restraint of Cdc7p signaling in early mitosis is unknown at present. It may be mediated by altering substrate availability or inhibition of the spindle pole body—bound fraction of the enzyme. For further details, see Discussion.