Evidence for defects in membrane traffic in Paramecium secretory mutants unable to produce functional storage granules

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Abstract
The ciliated protozoan Paramecium has a regulated secretory system amenable to genetic analysis. The secretory storage granules, known as trichocysts, enclose a crystalline matrix with a genetically determined shape whose biogenesis involves proteolytic maturation of a family of precursor molecules into a heterogeneous set of small acidic polypeptides that crystallize within the maturing vesicles. We have developed an original pulse-chase protocol for monoxenic Paramecium cultures using radiolabeled bacteria to study the processing of trichocyst matrix proteins in wild-type and mutant cells. In wild-type cells, proteolytic processing is blocked in the presence of monensin and otherwise rapidly completed after approximately 20 min of chase, suggesting that the conversion occurs in the trans-Golgi and/or in small vesicles soon after sorting to the regulated pathway, probably before crystallization begins. In trichless mutant cells, which contain no visible trichocysts, secretory proteins are synthesized but not processed and we report constitutive secretion of the uncleaved precursor molecules. The mutation thus appears to affect sorting to the regulated pathway and should prove useful for analysis of the sorting machinery and of the relationship between sorting and proteolytic processing of secretory proteins. In mutants bearing misshapen trichocysts with poorly crystallized contents (tam33, tam38, stubbyA), the proteolytic processing of the trichocyst matrix proteins appears to be normal, while both pulse-chase and morphological data indicate that intracellular transport is perturbed, probably between ER and Golgi. Precursor molecules are present in the mutant trichocysts but not in wild-type trichocysts and may account for the defective crystallization. Our analysis of these mutants suggests that the temporal coordination of intracellular traffic plays a regulatory role in granule maturation.