A LARGE MULTIGENE FAMILY CODES FOR THE POLYPEPTIDES OF THE CRYSTALLINE TRICHOCYST MATRIX IN PARAMECIUM

The secretory granules (trichocysts) of Paramecium are characterized b y a highly constrained shape that reflects the crystalline organizatio n of their protein contents. Yet the crystalline trichocyst content is composed not of a single protein but of a family of related polypepti des that derive from a family of precursors by protein processing. In this paper we show that a multigene family, of unusually large size fo r a unicellular organism, codes for these proteins. The family is orga nized 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 subfam ilies we have characterized are located on different macronuclear chro mosomes. 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 micro heterogeneity of structural proteins necessary for morphogenesis of a complex secretory granule core with a constrained shape and dynamic pr operties: genetic analysis has shown that correct assembly of the crys talline core is necessary for trichocyst function.