Amino acid sequences of lysozymes newly purified from invertebrates imply wide distribution of a novel class in the lysozyme family

Lysozymes were purified from three invertebrates: a marine bivalve, a marin e conch. and an earthworm. The purified lysozymes all showed a similar mole cular weight of 13 kDa on SDS/PAGE. Their N-terminal sequences up to the 33 rd residue determined here were apparently homologous among them; in additi on, they had a homology with a partial sequence of a starfish lysozyme whic h had been reported before. The complete sequence of the bivalve lysozyme w as determined by peptide mapping and subsequent sequence analysis. This was composed of 123 amino acids including as many as 14 cysteine residues and did not show a clear homology with the known types of lysozymes. However, t he homology search of this protein on the protein or nucleic acid database revealed two homologous proteins. One of them was a gene product, CELF22 A3 .6 of C. elegans, which was a functionally unknown protein. The other was a n isopeptidase of a medicinal leech, named destabilase. Thus, a new type of lysozyme found in at least four species across the three classes of the in vertebrates demonstrates a novel class of protein/lysozyme family in invert ebrates. The bivalve lysozyme, first characterized here, showed extremely h igh protein stability and hen lysozyme-like enzymatic features.