Crystal structure of the 30 K protein from the silkworm Bombyx mori reveals a new member of the β-trefoil superfamily

The hemolymph of the fifth instar larvae of the silkworm Bombyx mod contains a group of homologous proteins with a molecular weight of approximately 30 kDa, termed B. mod low molecular weight lipoproteins (Bmlps), which account for about 5% of the total plasma proteins. These so-called 30 K proteins have been reported to be involved in the innate immune response and transportation of lipid and/or sugar. To elucidate their molecular functions, we determined the crystal structure of a 30 K protein, Bmlp7, at 1.91 angstrom. It has two distinct domains: an all-alpha N-terminal domain (NTD) and an all-beta C-terminal domain (CTD) of the beta-trefoil fold. Comparative structural analysis indicates that Bmlp7 represents a new family, adding to the 14 families currently identified, of the beta-trefoil superfamily. Structural comparison and simulation suggest that the NTD has a putative lipid-binding cavity, whereas the CTD has a potential sugar-binding site. However, we were unable to detect the binding of either lipid or sugar. Therefore, further investigations are needed to characterize the molecular function of this protein. (C) 2011 Elsevier Inc. All rights reserved.