An N-acetyllactosamine-specific lectin, PFA, isolated from a moth (Phalera flavescens), structurally resembles an invertebrate-type lysozyme

Identification and Characterization of Genes for the Allo A Lectins in Japanese Rhinoceros Beetle (Trypoxylus dichotomus [Allomyrina dichotoma])

The Japanese rhinoceros beetle (Trypoxylus dichotomus [Allomyrina dichotoma]) produces the lectins allo A-I and allo A-II, which have strong N-acetyllactosamine (Galβ1-4GlcNAc)-binding activity. It has been suggested that the two lectins are formed from three subunits (α, β, and γ), with allo A-I comprising α and γ subunits and allo A-II comprising β and γ subunits. Here, we determined the cDNA sequences of these subunits using both conventional polymerase chain reaction (PCR)-cloning-sequencing and transcriptome-sequencing analyses. For the α and β subunits, one gene (locus) for each was predicted, whereas for the γ subunit, two types of cDNA sequences were obtained, which we named γ1 and γ2. These two types probably have distinct loci. Average nucleotide sequence identities among the subunits ranged from 87.6% (between α and γ1) to 92.6% (between γ1 and γ2), suggesting that they form a gene family. Although no homology was found between the sequences of allo A and other known lectin proteins in a protein database search, some unknown proteins containing the DUF3421 domain were identified. Those DUF3421 domain-encoding proteins are upregulated in the insect larval midgut. Thus, we infer that allo A genes also play an important role in larvae and that their lectin activity may have been obtained collaterally.

Expression and characterization of a recombinant i-type lysozyme from the harlequin ladybird beetle Harmonia axyridis

Lysozymes are enzymes that destroy bacterial cell walls by hydrolysing the polysaccharide component of peptidoglycan. In insects, there are two classes of lysozymes, the c-type with muramidase activity and the i-type whose prototypical members from annelids and molluscs possess both muramidase and isopeptidase activities. Many insect genes encoding c-type and i-type lysozymes have been identified during genome and transcriptome analyses, but only c-type lysozymes have been functionally characterized at the protein level. Here we produced one of five i-type lysozymes represented in the immunity-related transcriptome of the invasive harlequin ladybird beetle Harmonia axyridis as recombinant protein. This was the only one containing the serine and histidine residues that are thought to be required for isopeptidase activity. This i-type lysozyme was recombinantly expressed in the yeast Pichia pastoris, but the purified protein was inactive in both muramidase and isopeptidase assays. Transcription and immunofluorescence analysis revealed that this i-type lysozyme is produced in the fat body but is not inducible by immune challenge. These data suggest that i-type lysozymes in insects may have acquired novel and as yet undetermined functions in the course of evolution.