Fig. 3. How the SIN is wired. This figure is a representation of how the principal components of the SIN might interact. This is based upon localization studies, and the activity of proteins in different mutant backgrounds. For the sake of simplicity, the signal transduction event is portrayed as being linear, although there is no biochemical evidence to rule out a more complex arrangement, such as branching at the level of Spg1p. With the exception of the action of Byr4p-Cdc16p on Spg1p, direct regulation of one protein by another in vitro has not been demonstrated. Green arrows indicate septum-promoting events. Orange ball and stick symbols indicate the presumed point of action of negative regulators of SIN signaling. Inhibitors of SIN signaling are indicated in red, protein kinases in yellow, and phosphoprotein phosphatases in mauve. The roles and targets of Flp1p, Etd1p and Pld6p are unclear. Plo1p acts at or near the top of the network but its target has not been identified. Spg1p is shown twice, since it can form at least two complexes on the spindle pole body, one with Cdc7p, and the other with Byr4p-Cdc16p. Note that it is likely that Plo1p interacts with other spindle pole body components in addition to Cut12p. The large gray box represents the spindle pole body. The Mob1p-Sid2p complex is shown twice, once on the spindle pole body and once at the cell cortex, associated with the contractile actin ring. Zfs1 and Scw1 are shown downstream of the main SIN signaling module; their point of action is unclear.