Cloning and characterization of cDNAs encoding a novel cyclic AMP-binding protein in Dictyostelium discoideum.

The cellular slime mould, Dictyostelium discoideum, contains a novel cyclic AMP-binding protein, CABP1, which is composed of two subunits. Using anti-CABP1 monoclonal antibody as a probe, a cDNA clone was isolated from a lambda gt11 expression library. By hybrid selection of the complementary mRNA and its translation in vitro, we demonstrated that the cDNA hybridized to mRNAs encoding both CABP1 polypeptides. With the positive cDNA as a probe, we isolated a series of overlapping cDNA clones covering the coding region of both CABP1 mRNAs. Expression of the cloned cDNAs in bacteria and sequence analysis showed that the CABP1 subunits are identical in amino acid (aa) sequence, except that the small subunit is missing 37 aa near its N terminus. Genomic analysis suggested that the two CABP1 transcripts are derived from a single gene. The N-terminal half of each subunit is rich in proline, glutamine and glycine residues and contains a large block of aa repeats. The C-terminal half has an approx. 47% aa identity (86% with functionally conservative substitutions) with two polypeptides encoded by a plasmid determinant for tellurium anion resistance.