Spg1p. Spg1p (septum-promoting GTPase) was first identified in the fission yeast S. pombe, where it regulates septum formation in the septation initiation network (SIN) through the cdc7 protein kinase. Spg1p is an essential gene that localizes to the spindle pole bodies. When GTP-bound, it binds cdc7 and causes it to translocate to spindle poles. Sid4p (septation initiation defective) is required for localization of Spg1p to the spindle pole body, and the ability of Spg1p to promote septum formation from any point in the cell cycle depends on Sid4p. Spg1p is negatively regulated by Byr4 and cdc16, which form a two-component GTPase activating protein (GAP) for Spg1p. The existence of a SIN-related pathway in plants has been proposed. GTPase activating proteins (GAPs) interact with GTP-bound Rab and accelerate the hydrolysis of GTP to GDP. Guanine nucleotide exchange factors (GEFs) interact with GDP-bound Rabs to promote the formation of the GTP-bound state. Rabs are further regulated by guanine nucleotide dissociation inhibitors (GDIs), which facilitate Rab recycling by masking C-terminal lipid binding and promoting cytosolic localization.