Protein secretion in Tetrahymena thermophila. Characterization of the major proteinaceous secretory proteins

The contents of mucocysts of the ciliated protozoan Tetrahymena thermophila comprise about 12 proteins, ranging in relative mobility (M(r)) from approximately 160,000 to 8,000. There are at least four families of sulfhydryl-linked mucocyst polypeptides. One of these families includes a prominent M(r) 34,000 protein, as determined by one- and two-dimensional gel electrophoresis. The M(r) 34,000 protein is resolved into two species in isoelectric focusing gels, with apparent pI values of 4.8 and 4.9; most of the other mucocyst proteins also exhibit acidic apparent isoelectric points. The identity of the major M(r) 34,000 protein as a bona fide mucocyst component is substantiated by indirect immunofluorescent localization of this protein in a linear punctate pattern coincident with the localization of mucocysts in these cells; this pattern of localization can be abolished by stimulation of synchronous secretion and is absent in a mutant strain devoid of these secretory organelles (Maihle, N.J., and Satir, B.H. (1985a) J. Cell Sci. 78, 49-65).