Understanding the evolutionary structural variability and target specificity of tick salivary Kunitz peptides using next generation transcriptome data

Alexandra Schwarz,Jan Kopecký,James J Valdés,Alejandro Cabezas-Cruz

doi:10.1186/1471-2148-14-4

Alexandra Schwarz, Jan Kopecký + Show 2 more

Open Access

https://doi.org/10.1186/1471-2148-14-4

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Abstract

BackgroundTicks are blood-sucking arthropods and a primary function of tick salivary proteins is to counteract the host’s immune response. Tick salivary Kunitz-domain proteins perform multiple functions within the feeding lesion and have been classified as venoms; thereby, constituting them as one of the important elements in the arms race with the host. The two main mechanisms advocated to explain the functional heterogeneity of tick salivary Kunitz-domain proteins are gene sharing and gene duplication. Both do not, however, elucidate the evolution of the Kunitz family in ticks from a structural dynamic point of view. The Red Queen hypothesis offers a fruitful theoretical framework to give a dynamic explanation for host-parasite interactions. Using the recent salivary gland Ixodes ricinus transcriptome we analyze, for the first time, single Kunitz-domain encoding transcripts by means of computational, structural bioinformatics and phylogenetic approaches to improve our understanding of the structural evolution of this important multigenic protein family.ResultsOrganizing the I. ricinus single Kunitz-domain peptides based on their cysteine motif allowed us to specify a putative target and to relate this target specificity to Illumina transcript reads during tick feeding. We observe that several of these Kunitz peptide groups vary in their translated amino acid sequence, secondary structure, antigenicity, and intrinsic disorder, and that the majority of these groups are subject to a purifying (negative) selection. We finalize by describing the evolution and emergence of these Kunitz peptides. The overall interpretation of our analyses discloses a rapidly emerging Kunitz group with a distinct disulfide bond pattern from the I. ricinus salivary gland transcriptome.ConclusionsWe propose a model to explain the structural and functional evolution of tick salivary Kunitz peptides that we call target-oriented evolution. Our study reveals that combining analytical approaches (transcriptomes, computational, bioinformatics and phylogenetics) improves our understanding of the biological functions of important salivary gland mediators during tick feeding.

Highlights

Ticks are blood-sucking arthropods and a primary function of tick salivary proteins is to counteract the host’s immune response
Organizing the different I. ricinus salivary gland (SG) Kunitz peptides based on their Cys motifs facilitates functional predictions From the 454/Illumina SG I. ricinus transcriptome [18], we found archetypal Kunitz peptides containing 6 Cys residues and variants containing an additional 7th Cys residue adjacent to the conserved 5th Cys residue (Figure 1)
We infer that two main evolutionary pressures may be driving the evolution in I. ricinus Kunitz peptides – i.e., immune response and the necessity of functional diversity against the diverse host molecular targets

Summary

Introduction

Ticks are blood-sucking arthropods and a primary function of tick salivary proteins is to counteract the host’s immune response. The two main mechanisms advocated to explain the functional heterogeneity of tick salivary Kunitz-domain proteins are gene sharing and gene duplication. Both do not, elucidate the evolution of the Kunitz family in ticks from a structural dynamic point of view. As obligate hematophagous (blood-sucking) arthropods, ticks transmit various bacterial and viral diseases, e.g., babesiosis, theileriosis, anaplasmosis, Lyme disease and tick-borne projects from arthropod disease vectors are using Sanger or generation sequencing techniques These include several salivary gland (SG) transcriptome and proteome projects ( known as sialomes) from hard tick [6,7,8,9,10,11,12,13,14,15,16,17,18] and soft tick species [19,20,21,22]. These Kunitz peptides vary in their cysteine (Cys) motifs (possessing more or less than 6 Cys residues) with some lacking the archetypal disulfide bonds causing a more flexible fold, diversifying their inhibitory activity [26,32,40]

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