Isolation and characterization of proteins from the cuticle of Limulus polyphemus L

The gnathobasic spine microstructure of recent and Silurian chelicerates and the Cambrian artiopodan Sidneyia: Functional and evolutionary implications

Gnathobasic spines are located on the protopodal segments of the appendages of various euarthropod taxa, notably chelicerates. Although they are used to crush shells and masticate soft food items, the microstructure of these spines are relatively poorly known in both extant and extinct forms. Here we compare the gnathobasic spine microstructures of the Silurian eurypterid Eurypterus tetragonophthalmus from Estonia and the Cambrian artiopodan Sidneyiainexpectans from Canada with those of the Recent xiphosuran chelicerate Limulus polyphemus to infer potential variations in functional morphology through time. The thickened fibrous exocuticle in L. polyphemus spine tips enables effective prey mastication and shell crushing, while also reducing pressure on nerve endings that fill the spine cavities. The spine cuticle of E. tetragonophthalmus has a laminate structure and lacks the fibrous layers seen in L. polyphemus spines, suggesting that E. tetragonophthalmus may not have been capable of crushing thick shells, but a durophagous habit cannot be precluded. Conversely, the cuticle of S. inexpectans spines has a similar fibrous microstructure to L. polyphemus, suggesting that S. inexpectans was a competent shell crusher. This conclusion is consistent with specimens showing preserved gut contents containing various shelly fragments. The shape and arrangement of the gnathobasic spines is similar for both L. polyphemus and S. inexpectans, with stouter spines in the posterior cephalothoracic or trunk appendages, respectively. This differentiation indicates that crushing occurs posteriorly, while the gnathobases on anterior appendages continue mastication and push food towards and into the mouth. The results of recent phylogenetic analyses that considered both modern and fossil euarthropod clades show that xiphosurans and eurypterids are united as crown-group euchelicerates, with S. inexpectans placed within more basal artiopodan clades. These relationships suggest that gnathobases with thickened fibrous exocuticle, if not homoplasious, may be plesiomorphic for chelicerates and deeper relatives within Arachnomorpha. This study shows that the gnathobasic spine microstructure best adapted for durophagy has remained remarkably constant since the Cambrian.

Comprehensive sequence analysis of horseshoe crab cuticular proteins and their involvement in transglutaminase-dependent cross-linking

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.

Cuticular proteins from the horseshoe crab, Limulus polyphemus

Proteins were purified from the carapace cuticle of a juvenile horseshoe crab, Limulus polyphemus, and several of them were characterized by amino acid sequence determination. The proteins are small (7-16 kDa) and their isoelectric points range from 6.5 to 9.2. They have high contents of tyrosine, ranging from 13.5 to 35.4%. Some of the proteins show sequence similarity to cuticular proteins from other arthropod groups, with the most pronounced similarity to proteins from the cuticle of the spider Araneus diadematus. Two proteins show sequence similarity to a hexamerin storage protein from Blaberus discoidalis.