FIG 11.

Model for proteolytic processing of mucocyst proGrl proteins in Tetrahymena. We propose that processing of proGrl proteins during mucocyst maturation initiates with endoproteolytic cleavage by Cth3p at a site upstream of the mature N termini. The products are competent to form highly ordered complexes that assemble to create the elongated mucocyst core. Cth4p subsequently acts to trim the amino termini. This step is proposed to be essential for reinforcing the lattice in a way that promotes stability during expansion. Since removal of the peptides results in stabilization during expansion, we have termed them “destabilizing peptides” (DP). The relative timing of DP removal versus assembly is unknown. Cores that have not undergone DP trimming are defective during the directional expansion required for efficient core extrusion during exocytosis.