PROTEOLYTIC PROCESSING AND CA2-BINDING ACTIVITY OF DENSE-CORE VESICLEPOLYPEPTIDES IN TETRAHYMENA()

Formation and discharge of dense-core secretory vesicles depend on con trolled rearrangement of the core proteins during their assembly and d ispersal. The ciliate Tetrahymena thermophila offers a simple system i n which the mechanisms may be studied. Here we show that most of the c ore consists of a set of polypeptides derived proteolytically from fiv e precursors. These share little overall amino acid identity but are n onetheless predicted to have structural similarity. In addition, sites of proteolytic processing are notably conserved and suggest that spec ific endoproteases as well as carboxypeptidase are involved in core ma turation. Ln vitro binding studies and sequence analysis suggest that the polypeptides bind calcium in vivo. Core assembly and postexocytic dispersal are compartment-specific events. Two likely regulatory facto rs are proteolytic processing and exposure to calcium. We asked whethe r these might directly influence the conformations of core proteins. R esults using an in vitro chymotrypsin accessibility assay suggest that these factors can induce sequential structural rearrangements. Such p rogressive changes in polypeptide folding may underlie the mechanisms of assembly and of rapid postexocytic release. The parallels between d ense-core vesicles in different systems suggest that similar mechanism s are widespread in this class of organelles.