No abstract

The arthropod cuticle functions principally as an exoskeleton covering the total body surface, and is a highly organized structure produced by extracellular secretion from the epidermis. It is constructed as a composite consisting of chitin filaments (a homopolymer of N-acetyl glucosamines conjugated by b-1,4 linkages), structural proteins, lipids, catecholamine derivatives, and minerals. Its structural properties, however, vary among species and also according to surface location and developmental stage in individuals [1][2][3]. The mechanical properties of the cuticle depend on the content of chitin, the microarchitecture of chitin filaments, and the interaction between the chitinfilament system and cuticular proteins. Furthermore, the cuticle can be modified by sclerotization, namely the oxidative incorporation of o-diphenols into cuticular Arthropod cuticles play an important role as the first barrier against invading pathogens. We extensively determined the sequences of horseshoe crab cuticular proteins. Proteins extracted from a part of the ventral side of the cuticle were purified by chitin-affinity chromatography, and separated by two-dimensional SDS ⁄ PAGE. Proteins appearing on the gel were designated high molecular mass chitin-binding proteins, and these proteins were then grouped into classes based on their approximate isoelectric points and predominant amino acid compositions. Members of groups designated basic G, basic Y, and acidic S groups contained a so-called Rebers and Riddiford consensus found in arthropod cuticular proteins. Proteins designated acidic DE25 and DE29 each contained a Cys-rich domain with sequences similar to those of insect peritrophic matrix proteins and chitinases. In contrast, basic QH4 and QH10 contained no consensus sequences found in known chitin-binding proteins. Alternatively, a low molecular mass chitin-binding fraction was prepared by size exclusion chromatography, and 15 low molecular mass chitin-binding proteins, named P1 through P15, were isolated. With the exception of P9 and P15, all were found to be identical to known antimicrobial peptides. P9 consisted of a Kunitz-type chymotrypsin inhibitor sequence, and P15 contained a Cys-rich motif found in insulin-like growth factor-binding proteins. Interestingly, we observed transglutaminase-dependent polymerization of nearly all high molecular mass chitin-binding proteins, a finding suggests that transglutaminase-dependent cross-linking plays an important role in host defense in the arthropod cuticle, analogous to that observed in the epidermal cornified cell envelope in mammals.

In the horseshoe crab, the recognition of β-1,3-d-glucans by factor G triggers hemolymph coagulation. Factor G contains a domain of two tandem xylanase Z-like modules (Z1-Z2), each of which recognizes β-1,3-d-glucans. To gain an insight into the recognition of β-1,3-d-glucans from a structural view point, recombinants of Z1-Z2, the C-terminal module Z2, Z2 with a Cys to Ala substitution (Z2A), and its tandem repeat Z2A-Z2A were characterized. Z2 and Z1-Z2, but not Z2A and Z2A-Z2A, formed insoluble aggregates at higher concentrations more than ∼30 and 3 μM, respectively. Z1-Z2 and Z2A-Z2A bound more strongly to an insoluble β-1,3-d-glucan (curdlan) than Z2A. The affinity of Z2A for a soluble β-1,3-d-glucan (laminarin) was equivalent to those of Z1-Z2, Z2A-Z2A, and native factor G, suggesting that the binding of a single xylanase Z-like module prevents the subsequent binding of another module to laminarin. Interestingly, Z2A as well as intact factor G exhibited fungal agglutinating activity, and fungi were specifically detected with fluorescently tagged Z2A by microscopy. The chemical shift perturbation of Z2A induced by the interaction with laminaripentaose was analyzed by nuclear magnetic resonance spectroscopy. The ligand-binding site of Z2A was located in a cleft on a β-sheet in a predicted β-sandwich structure, which was superimposed onto cleft B in a cellulose-binding module of endoglucanase 5A from the soil bacterium Cellvibrio mixtus. We conclude that the pattern recognition for β-1,3-d-glucans by factor G is accomplished via a carbohydrate-binding cleft that is evolutionally conserved between horseshoe crab and bacteria.