Immature secretory granules are not acidic in Paramecium: Implications for sorting to the regulated pathway

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Abstract
Summary— Paramecium, like other ciliates, has a system of regulated secretion. The secretory storage granules, known as trichocysts, are of large size and elaborate architecture and occupy in wild type cells a significant fraction of the cellular volume. They thus provide an excellent model system for studies of secretory granule biogenesis. Previous analysis of secretory mutants unable to produce functional trichocysts (Gautier et al (1994). J Cell Biol 124, 893–902) suggested that one of the mutants studied, trichless, might have defective organelle acidification. We have therefore applied a technique (Anderson et al (1984) Proc Natl Acad Sci USA 81, 4838–4842) for the postembedding characterization of acidic compartments using the weak base 3‐(2,4‐dinitroanilino)‐3′‐amino‐N‐methyldipropylamine (DAMP) to Paramecium. Quantitative immunolocalization experiments using double label (antibodies against DAMP and against regulated secretory proteins) show that secretory granules are not significantly acidic at any stage of their development in Paramecium. Analysis of trichless mutant cells shows that organelle acidification is not defective and allows us to follow the regulated secretory proteins from apparently normal immature secretory granules to a probable degradative compartment. Consideration of the ensemble of the results leads us to discuss the possibility that immature secretory granules are a sorting compartment in Paramecium, as originally proposed by Kuliawat and Arvan ((1994) J Cell Biol 126, 77–86) for the insulin producing β‐cells of pancreatic islets.

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