ISOLATION AND CHARACTERIZATION OF 2 GENES ENCODING CALITOXINS, NEUROTOXIC PEPTIDES FROM CALLIACTIS-PARASITICA (CNIDARIA)

Among sea anemone neurotoxins, calitoxin, recently isolated from Calli actis parasitica, is a highly toxic peptide of 46 amino acids (aa), wh ose sequence differs greatly from that of all sea anemone toxins isola ted so far. In this study, two genes (clx-1 and clx-2) coding for two highly homologous calitoxins were isolated and characterized from a C. parasitica genomic library. The clx-1 gene encodes the already known calitoxin sequence, named CLX-I, whereas a single bp substitution in t he coding region of clx-2 is responsible for a single Glu(6) --> Lys r eplacement in a new peptide named CLX-II. The structural organization of the two genes is very similar: two introns and three exons, whose s equences are highly homologous for clx-1 and clx-2 (95% identity). The open reading frame (ORF) of both clx-1 and clx-2 codes for a precurso r peptide of 79 aa, whose N-terminus has the feature of a signal pepti de, while the C-terminus corresponds to the sequences of mature CLX-I and CLX-II. The finding that a pair of basic aa is located upstream fr om the sequence of both mature toxins strongly suggests that proteolyt ic events, at specific cleavage sites, are responsible for the release of neurotoxins from their respective precursor molecules.