PROTEIN PROCESSING AND MORPHOGENESIS OF SECRETORY GRANULES IN PARAMECIUM

The ciliated protozoan Paramecium provides a model system for the stud y of regulated secretion, featuring architecturally complex secretory storage granules - trichocysts - docked at the plasma membrane, ready to respond to an exocytotic stimulus. The trichocysts are characterize d by crystalline contents that confer upon the organelle a defined sha pe which can be altered by single gene mutation. The crystalline trich ocyst contents are built up from a heterogeneous set of small acidic p olypeptides generated by proteolytic maturation of a family of precurs or molecules, suggesting an important role for protein processing in t his system. We have recently shown that the primary defect in several secretory mutants lacking functional trichocysts is in intracellular t rafficking rather than protein processing. However, analysis of how th ese defects lead to altered trichocyst shape supports the notion that the protein processing is essential for morphogenesis. Preliminary res ults of a cloning project reveal that an extensive multigene family (s imilar to 100 genes) codes for the trichocyst matrix proteins. Deduced amino acid sequences of putative processing sites indicate that (at l east) two distinct processing reactions are probably involved in the m aturation of these proteins, and allow us to speculate that each react ion may control a key event of trichocyst biogenesis.