Tapes japonica lysozyme (TJL) is classified as a member of the recently established i-type lysozyme family. In this study, we solved the crystal structure of TJL complexed with a trimer of N-acetylglucosamine to 1.6 Å resolution. Based on structure and mutation analyses, we demonstrated that Glu-18 and Asp-30 are the catalytic residues of TJL. Furthermore, the present findings suggest that the catalytic mechanism of TJL is a retaining mechanism that proceeds through a covalent sugar-enzyme intermediate. On the other hand, the quaternary structure in the crystal revealed a dimer formed by the electrostatic interactions of catalytic residues (Glu-18 and Asp-30) in one molecule with the positive residues at the C terminus in helix 6 of the other molecule. Gel chromatography analysis revealed that the TJL dimer remained intact under low salt conditions but that it dissociated to TJL monomers under high salt conditions. With increasing salt concentrations, the chitinase activity of TJL dramatically increased. Therefore, this study provides novel evidence that the lysozyme activity of TJL is modulated by its quaternary structure.A known bacteriolytic enzyme, lysozyme (EC 3.2.1.17), is widely distributed throughout the animal and plant kingdom. Several types of lysozyme have been described to date as follows: chicken, goose, bacteria, plant, and phage. Recently, there has been increasing interest in a new type of lysozyme, i.e. invertebrate-type (i-type) 4 lysozyme. The existence of this new type of lysozyme was proposed as early as 1975 (1). A lysozyme composed of 123 amino acids (13.8 kDa) was recently isolated from the marine bivalve Tapes japonica and was shown to be an i-type lysozyme (2). This was the first lysozyme to be identified as an i-type lysozyme based on determination of the complete amino acid sequence at the protein level. Furthermore, several lysozymes have been identified, including those from the following organisms: two coastal bivalves belonging to the genus Mytilus (3), four deep-sea bivalves belonging to the genera Bathymodiolus and Calyptogena (3), a bivalve belonging to Chamys islandica (4, 5), and a starfish belonging to Asterias rubens (6). In 2002, alignment and phylogenic analyses using six bivalve lysozymes suggested that i-type lysozymes form a monophyletic family (7). The tertiary structures of representative lysozymes from chickens (8), geese (9), bacteria (10), plants (11), and phages (12) have already been determined but that of the i-type lysozymes has not. Because the primary sequences of i-type lysozymes are not homologous to those of any other types of lysozyme, the tertiary structure cannot be constructed. Interestingly, the lytic activity of bivalve lysozymes has been shown to be very sensitive to the ionic strength of a solution. With increasing salt concentrations, oyster and blue mussel lysozyme activity against Micrococcus lysodeikticus was activated and increased until exposure to a 0.1 M salt solution had been reached; at higher salt concentrations, lysozymatic activity t...