Glycosylated triterpenoid saponin: A specific inhibitor of diguanylate cyclase from Acetobacter xylinum. Biological activity and distribution

In a recent paper (Ohana et al. 1998), we described the purification and structural characterization of a novel glycosidic triterpenoid saponin (GTS), a specific inhibitor of diguanylate cyclase (dgc), the key regulatory enzyme of the cellulose synthesizing apparatus of the bacterium Acetobacter xylinum. This compound and an identical or very similar one were isolated from pea (Pisum sativum), and A. xylinum respectively. We now present the effects of GTS on the kinetic properties of dgc. The observed inhibition is non-competitive with respect to the substrate GTP, is decreased by 50% in the presence of 20 μM c-di-GMP, and is not observed in the presence of various detergents. Photo-affinity labeling studies using [³²P]c-di-GMP and purified enzyme showed that the inhibitor affects binding of c-diGMP to dgc. It was ascertained that GTS, or at least a very similar compound, is present in other plant systems. The antibiotic Papulacandin B also inhibits dgc, although less efficiently than GTS (Ki = 70 μM vs. 5 μM). In in situ assays, highly purified GTS inhibits bacterial cellulose synthesis and dgc activity. Similarly, digitonin permeabilized tobacco cell cultures display inhibition of glucan synthesis in the presence of GTS.