A kinetically stable plant subtilase with unique peptide mass fingerprints and dimerization properties

Milin, a potent molluscicide from the latex of Euphorbia milii, holds promise in medicinal biochemistry. Electrophoresis, size exclusion chromatography, mass spectrometry and other biochemical characteristics identify milin as a homodimeric, plant subtilisin-like serine protease, the first of its kind. The subunits of milin are differentially glycosylated affecting dimer association, solubility and proteolytic activity. The dimeric dissociation is SDS-insensitive and strongly temperature dependent but does not appear to be linked by disulfide bridges. N-terminal sequence of acid hydrolyzed peptide fragments shows no homology to known serine protease. Peptide mass fingerprinting and de novo sequencing of the tryptic fragments also did not identify putative domains in the protein. Milin seems to be a novel plant enzyme with subunit association partly similar to human herpes virus serine proteases and partly to penicillin binding proteins. Its behaviour on SDS-PAGE gels and other properties is like "kinetically stable" proteins. Such subunit association and properties might play a critical role in its physiological function and in controlling Schistosomiasis.