Molecular cloning and characterization of the cDNA for discoidin II of Dictyostelium discoideum

By using monoclonal antibodies directed against discoidin II, we have isolated cDNA clones from axenically grown Ax-2 cells. One cDNA clone (D2) contained a 1,2-kb insert encoding the entire discoidin II protein, which is composed of 257 amino acid residues and has a calculated molecular mass of 28,574. The amino acid sequences, determined by Edman degradation of six tryptic peptides of discoidin II, were identical to those deduced from the cDNA sequence. The protein bears no resemblance to any proteins in the data banks, except that its sequence is 49% identical with the amino acid sequence of discoidin I. Discoidin II shares with discoidin I both a carbohydrate-binding site and an Arg-Gly-Asp (RGD) sequence, which has been found in fibronectin in mammalian cells. With the onset of aggregation (8h of development), a 1,3-kb discoidin II mRNA begins to accumulate. A similar pattern of regulation occurs at the protein level.