Characterisation of protein families in spider digestive fluids and their role in extra-oral digestion

Abstract

Background: Spiders are predaceous arthropods that are capable of subduing and consuming relatively large prey items compared to their own body size. For this purpose, spiders have evolved potent venoms to immobilise prey and digestive fluids that break down nutrients inside the prey's body by means of extra-oral digestion (EOD). Both secretions contain an array of active proteins, and an overlap of some components has been anecdotally reported, but not quantified. We systematically investigated the extent of such protein overlap. As venom injection and EOD succeed each other, we further infer functional explanations, and, by comparing two spider species belonging to different clades, assess its adaptive significance for spider EOD in general.

Results: We describe the protein composition of the digestive fluids of the mygalomorph Acanthoscurria geniculata and the araneomorph Stegodyphus mimosarum, in comparison with previously published data on a third spider species. We found a number of similar hydrolases being highly abundant in all three species. Among them, members of the family of astacin-like metalloproteases were particularly abundant. While the importance of these proteases in spider venom and digestive fluid was previously noted, we now highlight their widespread use across different spider taxa. Finally, we found species specific differences in the protein overlap between venom and digestive fluid, with the difference being significantly greater in S. mimosarum compared to A. geniculata.

Conclusions: The injection of venom precedes the injection with digestive fluid, and the overlap of proteins between venom and digestive fluid suggests an early involvement in EOD. Species specific differences in the overlap may reflect differences in ecology between our two study species. The protein composition of the digestive fluid of all the three species we compared is highly similar, suggesting that the cocktail of enzymes is highly conserved and adapted to spider EOD.

Keywords: Acanthoscurria; Astacin metalloproteases; Digestive fluid; Extra-oral digestion; Proteomics; Spider; Stegodyphus; Venom.

Fig. 1

Scheme of the succession of steps of extra-oral digestion (EOD) in spiders with reference to the enzymes involved

Fig. 2

Phylogenetic position of our study species Stegodyphus mimosarum and Acanthoscurria geniculata in comparison to a third species used in a methodologically similar study by Fuzita et al. [15], Nephilingis cruentata. The latest common ancestor of both of our study species lived around 380 million years ago, demonstrating the long-time of independent evolution

Fig. 3

Alignment of protein sequences of all astacin-like metalloproteases found in the genome of Stegodyphus mimosarum. Highlighted in red are four conserved cysteins, and in grey a conserved HEXXHXXGXXHE motif. Both the conserved cysteins and the HEXXHXXGXXHE motif are characteristic for astacin-like metalloproteases (Gomis-Ruth et al. [56])

Fig. 4

Genomic location of the astacin-like metalloproteases found in Stegodyphus mimosarum. Each locus is represented by black arrows pointing from 3′ to 5′ end. The total length of each scaffold is written to the right. The entire scaffolds are only shown if they are shorter than 320 kb. Astacins in red are present in both, digestive fluid and venom (cf. Additional file 3)

Fig. 5

Number of detected proteins in our study species S. mimosarum and A. geniculata with special respect to overlaps in the compositions of venom and digestive fluid. Left: All detected proteins; Right: Quantifiable proteins only. There is a significantly greater overlap in S. mimosarum (χ² = 89.959, df = 2, p < 0.001)