We isolated and sequenced the cDNAs coding for lysozymes of six bivalve species. Alignment and phylogenetic analysis showed that, together with recently described bivalve lysozymes, the leech destabilase, and a number of putative proteins from extensive genomic and cDNA analyses, they belong to the invertebrate type of lysozymes (i type), first described by Jollès and Jollès (1975). We determined the genomic structure of the gene encoding the lysozyme of Mytilus edulis, the common mussel. We provide evidence that the central exon of this gene is homologous to the second exon of the chicken lysozyme gene, belonging to the c type. We propose that the origin of this domain can be traced back in evolution to the origin of bilaterian animals. Phylogenetic analysis suggests that i-type proteins form a monophyletic family.

This study shows that PEBP is not related to the G-protein family nor to known lipid-binding proteins, and therefore defines a novel structural family of phospholipid-binding proteins. The PEBP structure contains no internal hydrophobic pocket, as described for lipocalins or small phospholipid-transfer proteins. Nevertheless, in PEBP, a small cavity close to the protein surface has a high affinity for anions, such as phosphate and acetate, and also phosphorylethanolamine. We suggest that this cavity corresponds to the binding site of the polar head group of phosphatidylethanolamine.