Cellular localization of the SerH surface antigen in Tetrahymena thermophila

We have purified the SerH surface antigen of T. thermophila by a novel and simple procedure and produced specific antisera against it. By the use of various immunochemical techniques, we have investigated the intracellular distribution of the antigen and shown that SerH is not only abundant in the cortex but also in the digestive apparatus of the ciliate. In each of these two localizations, SerH occupies a variety of compartments: in the cortex it can be found in the surface coat, the exocytotic mucocysts, the endocytotic parasomal sacs and as an integral protein of the membrane; in the digestive apparatus, SerH is found around ingested bacteria, in the cytoplasm surrounding the cytopharynx and the forming food vacuoles, and, seemingly, as a membrane-associated protein in young food vacuoles. Pulse-chase experiments have shown that feeding dramatically increases the global turnover of SerH. Besides these localizations, SerH is principally found in clumps around dense intracytoplasmic spheres, which could be mucocyst precursors. Indirect evidence is presented that SerH is routed to the peripheral or extracellular compartments via the mucocysts. The antigen is absent from alveolar, nuclear or mitochondrial membranes. We propose that SerH covers any membrane in contact or future contact with the extracellular medium.