Characterization of a Novel C-Type Lectin, Bombyx mori Multibinding Protein, from the B. mori Hemolymph: Mechanism of Wide-Range Microorganism Recognition and Role in Immunity

Abstract

To investigate the system used by insects to recognize invading microorganisms, we examined proteins from the larval hemolymph of Bombyx mori that bind to the cell surface of microorganisms. Two hemolymph proteins that bound to the cell surfaces of Micrococcus luteus and Saccharomyces cerevisiae were shown to be identical. This protein bound to all 11 microorganisms examined–5 Gram-negative bacteria, 3 Gram-positive bacteria, and 3 yeasts–and was consequently designated B. mori multibinding protein (BmMBP). The sequence of the cDNA encoding BmMBP revealed that it was a C-type lectin with two dissimilar carbohydrate-recognition domains (CRD1 and CRD2) distantly related to known insect C-type lectins. CRD1 and CRD2 were prepared as recombinant proteins and their binding properties were investigated using inhibition assays. Each domain had wide, dissimilar binding spectra to sugars. These properties enable BmMBP to bind to two sites on a microorganism, facilitating high-affinity binding to many types of microorganisms. The dissociation constants of BmMBP with M. luteus cells and S. cerevisiae were 1.23 × 10−8 and 1.00 × 10−11 M, respectively. rBmMBP triggered the aggregation of hemocytes from B. mori larvae in vitro and microorganisms recognized by BmMBP were surrounded by aggregated hemocytes in vivo, forming a nodule, which is the typical cellular reaction in insect immune responses. These observations suggest that BmMBP functions as a trigger for the nodule reaction and that the multirecognition characteristic of BmMBP plays an important role in the early stages of infection by a variety of microorganisms.