Monitoring Editor: David G. Drubin Spore morphogenesis in yeast is driven by the formation of membrane compartments that initiate growth at the spindle poles during Meiosis II and grow to encapsulate daughter nuclei. Vesicle docking complexes, called Meiosis II Outer Plaques (MOPs), form on each Meiosis II spindle pole body (SPB) and serve as sites of membrane nucleation. How the MOP stimulates membrane assembly is not known. Here, we report that SpSpo13, a component of the MOP in S. pombe, shares homology with the guanine nucleotide exchange factor (GEF) domain of the S. cerevisiae Sec2 protein. ScSec2 acts as a GEF for the small Rab GTPase ScSec4, which regulates vesicle trafficking from the late-Golgi to the plasma membrane. A chimeric protein in which the ScSec2-GEF domain is replaced with SpSpo13 is capable of supporting the growth of a sec2Delta mutant. SpSpo13 binds preferentially to the nucleotide-free form of ScSec4 and facilitates nucleotide exchange in vitro. In vivo, a Spspo13 mutant defective in GEF activity fails to support membrane assembly. In vitro specificity experiments suggests that SpYpt2 is the physiological substrate of SpSpo13. These results demonstrate that stimulation of Rab-GTPase activity is a property of the S. pombe MOP essential for the initiation of membrane formation.
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