Immunological characterization of trichocyst proteins in the ciliate Pseudomicrothorax dubius

Biogenesis of Dense-Core Secretory Granules

Dense core granules (DCGs) are vesicular organelles derived from outbound traffic through the eukaryotic secretory pathway. As DCGs are formed, the secretory pathway can also give rise to other types of vesicles, such as those bound for endosomes, lysosomes, and the cell surface. DCGs differ from these other vesicular carriers in both content and function, storing highly concentrated cores’ of condensed cargo in vesicles that are stably maintained within the cell until a specific extracellular stimulus causes their fusion with the plasma membrane. These unique features are imparted by the activities of membrane and lumenal proteins that are specifically delivered to the vesicles during synthesis. This chapter will describe the DCG biogenesis pathway, beginning with the sorting of DCG proteins from proteins that are destined for other types of vesicle carriers. In the trans-Golgi network (TGN), sorting occurs as DCG proteins aggregate, causing physical separation from non-DCG proteins. Recent work addresses the nature of interactions that produce these aggregates, as well as potentially important interactions with membranes and membrane proteins. DCG proteins are released from the TGN in vesicles called immature secretory granules (ISGs). The mechanism of ISG formation is largely unclear but is not believed to rely on the assembly of vesicle coats like those observed in other secretory pathways. The required cytosolic factors are now beginning to be identified using in vitro systems with purified cellular components. ISG transformation into a mature fusion-competent, stimulus-dependent DCG occurs as endoproteolytic processing of many DCG proteins causes continued condensation of the lumenal contents. At the same time, proteins that fail to be incorporated into the condensing core are removed by a coat-mediated budding mechanism, which also serves to remove excess membrane and membrane proteins from the maturing vesicle. This chapter will summarize the work leading to our current view of granule synthesis, and will discuss questions that need to be addressed in order to gain a more complete understanding of the pathway.

A large multigene family codes for the polypeptides of the crystalline trichocyst matrix in Paramecium

The secretory granules (trichocysts) of Paramecium are characterized by a highly constrained shape that reflects the crystalline organization of their protein contents. Yet the crystalline trichocyst content is composed not of a single protein but of a family of related polypeptides that derive from a family of precursors by protein processing. In this paper we show that a multigene family, of unusually large size for a unicellular organism, codes for these proteins. The family is organized in subfamilies; each subfamily codes for proteins with different primary structures, but within the subfamilies several genes code for nearly identical proteins. For one subfamily, we have obtained direct evidence that the different members are coexpressed. The three subfamilies we have characterized are located on different macronuclear chromosomes. Typical 23-29 nucleotide Paramecium introns are found in one of the regions studied and the intron sequences are more variable than the surrounding coding sequences, providing gene-specific markers. We suggest that this multigene family may have evolved to assure a microheterogeneity of structural proteins necessary for morphogenesis of a complex secretory granule core with a constrained shape and dynamic properties: genetic analysis has shown that correct assembly of the crystalline core is necessary for trichocyst function.

Immunological similarities among trichocyst secretory proteins of Paramecium caudatum, Paramecium tetraurelia and Pseudomicrothorax dubius

Secretory vesicles called trichocysts structurally show a body and a tip in Paramecium and a shaft and four arms in Pseudomicrothorax. Biochemical and immunological relationships between both trichocyst types were examined. Three polyclonal antibodies were produced against 3 groups of protein bands of isolated Pseudomicrothorax dubius trichocysts separated by one-dimensional gel electrophoresis: G 1 (30-31 kDa), G 2 (26-27 kDa) and G 3 (15-20 kDa). By indirect immunofluorescent binding assay, the three antisera label extended (i.e. discharged) and condensed (i.e. non-discharged) trichocysts of both Ps. dubius and Paramecium caudatum. By immunogold labeling on ultrathin sections, the three antisera label the trichocyst shaft of Ps. dubius and the trichocyst body of Pa. caudatum on both condensed and extended secretory vesicles. On two-dimensional immunoblots, remarkable antigenic similarities are shown by trichocysts of Ps. dubius, Paramecium tetraurelia nd6 and Pa. caudatum. Anti-G 1 serum detects proteins at 28-32 kDa, pI 5.0-5.7, in the three cells. Anti-G 3 serum labels one protein group at ~ 15-22 kDa and a second protein group at ~ 35-50 kDa in Ps. dubius, Pa. tetraurelia nd6 and Pa. caudatum. Anti-G 2 serum labels proteins at ~ 15-22 kDa, pI 4.7-5.2, in all three species. Anti-G 2 serum also detects bands at 24-28 kDa and 30-31 kDa, pI 4.7-6.1, in Ps. dubius. No equivalent M(r) proteins are observed in either Paramecium spp. Comparisons of immunoblots of proteins of entire cells with and without ejectable trichocysts allowed identification of non-ejectable trichocyst proteins, some of which may be precursors localized within pretrichocysts. Such proteins are at 41-47 kDa in Ps. dubius, and at ~ 45 kDa in Paramecium.