Immunity or Digestion

Abstract

The cell surfaces of microorganisms display distinct molecular patterns formed from lipopolysaccharides, peptidoglycans, or β1,3-glucans. Binding of these surfaces by pattern recognition proteins such as β1,3-glucan recognition proteins (βGRPs) activates the immune response in arthropods. We identified a 40-kDa β1,3-glucan-binding protein with sequence similarity to previously characterized lepidopteran βGRPs from hemolymph, but unlike these it is secreted into the larval gut lumen and is an active β1,3-glucanase. This glucanase was not detected in hemolymph. Its mRNA is constitutively and predominantly expressed in the midgut and is induced there when larvae feed on a diet containing bacteria. Homologs of this predominantly midgut-expressed gene from many Lepidoptera possess key residues shown to be part of the active site of other glucanases, and form a cluster that is distinct from previously described βGRPs. In addition, this group includes proteins from insects such as the Anopheles gambiae GNBP subgroup B for which a catalytic role has not been previously suspected. The current domain classification does not distinguish between the catalytic and noncatalytic clades, and should be revised. The noncatalytic βGRPs may be evolutionarily derived from this newly described enzyme family that continues to function catalytically in digestion and/or pathogen defense. The cell surfaces of microorganisms display distinct molecular patterns formed from lipopolysaccharides, peptidoglycans, or β1,3-glucans. Binding of these surfaces by pattern recognition proteins such as β1,3-glucan recognition proteins (βGRPs) activates the immune response in arthropods. We identified a 40-kDa β1,3-glucan-binding protein with sequence similarity to previously characterized lepidopteran βGRPs from hemolymph, but unlike these it is secreted into the larval gut lumen and is an active β1,3-glucanase. This glucanase was not detected in hemolymph. Its mRNA is constitutively and predominantly expressed in the midgut and is induced there when larvae feed on a diet containing bacteria. Homologs of this predominantly midgut-expressed gene from many Lepidoptera possess key residues shown to be part of the active site of other glucanases, and form a cluster that is distinct from previously described βGRPs. In addition, this group includes proteins from insects such as the Anopheles gambiae GNBP subgroup B for which a catalytic role has not been previously suspected. The current domain classification does not distinguish between the catalytic and noncatalytic clades, and should be revised. The noncatalytic βGRPs may be evolutionarily derived from this newly described enzyme family that continues to function catalytically in digestion and/or pathogen defense. Recognition of invading organisms as non-self is a crucial pre-requisite for an immune response. In arthropods, immune response is triggered in the hemolymph by so-called pattern recognition proteins or receptors, which bind to classes of bacteria- and fungi-specific polysaccharides such as peptidoglycans, β1,3-glucans, lipopolysaccharides (LPS), 3The abbreviations used are: LPS, lipopolysaccharide; LTA, lipotechoic acid; PGRP, peptidoglycan recognition protein; proPO, prophenoloxidase; GNBP, Gram-negative bacteria-binding protein; βGRP, β1,3-glucan recognition protein; PBS, phosphate-buffered saline; MALDI-TOF, matrix-assisted laser desorption ionization time-of-flight; MS, mass spectrometry; RT, reverse transcription. and lipotechoic acid (LTA) (1Kanost M.R. Jiang H. Yu X.Q. Immunol. Rev.. 2004; 198: 97-105Google Scholar, 2Medzhitov R. Janeway Jr., C.A. Cell.. 1997; 91: 295-298Google Scholar). Little detail is known for Lepidoptera but in Drosophila these binding interactions lead to activation of the innate immune response, largely regulated by two main pathways, the Toll and Imd pathways. The innate immune system contains of three main effector mechanisms: the cellular response, the humoral response, and