Inhibition of ATPase activity in pea plasma membranes by fungal suppressors from mycosphaerella pinodes and their peptide moieties

The effects of two suppressors of the defense reactions of host plants, which had been purified from the pea pathogen Mycosphaerella pinodes, as well as the effects of peptide moieties, on the ATPase activity in pea plasma membranes were examined in vitro. One of the suppressors, Supprescin B, inhibited the ATPase activity in a non-competitive manner, but the other suppressor, Supprescin A, did not. Supprescin A was observed to reduce the inhibitory effect of Supprescin B. A tripeptide, Ser-Ser-Gly, and a hexapeptide, Ser-Ser-Gly-Asp-Glu-Thr, which were the respective peptide moieties of Supprescin A and B, inhibited the ATPase activity in a competitive manner. Supprescin B and fragments of the hexapeptide, such as Asp-Glu-Thr and Gly- Asp-Glu, inhibited not only the ATPase activity but also the acid phosphatase activity of plasma membranes in vitro. These results indicate that the acidic amino-acid residues of the "Asp-Glu" moiety seem to act as inhibitors of the phosphatase activity. Thus, the peptide moiety of Supprescin B consists of at least two functional elements.