Secretion of spore coat proteins from the prespore secretory vesicles (PSVs) in 
Dictyostelium discoideum is a signal mediated event that underlies terminal cell 
differentiation, and represents an important case of developmentally regulated 
secretion. In order to study the biochemical mechanisms that govern the 
regulated fusion of the PSVs with the plasma membrane and the subsequent 
secretion of their cargo, we purified this organelle from prespore cells. 
Analysis of protein extracts of highly purified PSVs indicated that, in addition 
to the cargo of structural spore coat proteins, many more proteins are 
associated with the PSVs. Their identification is paramount to the understanding 
of the mechanism of regulated secretion in this system. In this study we have 
taken the first comprehensive proteomic approach to the analysis of an entire, 
previously uncharacterized, organelle, with the goal of identifying the major 
proteins associated with the PSVs. We show that in addition to the structural 
spore coat proteins, the PSVs contain the enzymes needed for proper spore coat 
assembly (thioredoxin 2 and 3), regulatory proteins which we predict receive and 
transduce the developmental signal for secretion (rab7 GTPase, PI-3 kinase, NDP 
kinase and the calcium binding proteins calfumirin-1 and calreticulin) as well 
as proteins that interact with the cytoskeleton to mediate movement of the PSVs 
to the plasma membrane (actin binding proteins coactosin and profilin 1). In 
addition, the results suggest that proteins can play multiple roles in the cell, 
and that protein function can be dictated in part by subcellular localization. 
The identification of the PSV proteins is allowing us to develop testable 
hypotheses about the roles of these proteins within the functional context of 
developmentally regulated secretion.