Recognition and binding of the PF2 lectin to α-amylase from Zabrotes subfasciatus (Coleoptera:Bruchidae) larval midgut

Characterization and expression of prohibitin during the mexican bean weevil (Zabrotes subfasciatus, Boheman, 1833) larvae development

Prohibitin (PHB) is a highly conserved eukaryotic protein complex involved in multiple cellular processes. In insects, PHB has been identified as a potential target protein to insecticidal molecules acting as a receptor of PF2 insecticidal lectin in the midgut of Zabrotes subfasciatus larvae (bean pest) and Cry protein of Bacillus thuringiensis in Leptinotarsa decemlineata (Colorado potato beetle). This study aimed to characterize the structural features of Z. subfasciatus prohibitin (ZsPHB) by homology modeling and evaluate its expression and tissue localization at different stages of larval development both at the transcript and protein levels. The samples were collected from eggs and larvae of different developmental stages. The immunodetection of ZsPHB was done with anti-PHB1 and confirmed by LC-MS/MS analysis. Gene expression analysis of ZsPHB1 and ZsPHB2 was performed by RT-qPCR, and immunohistochemistry with FITC-labeled anti-PHB1. Results showed that ZsPHBs exhibit distinctive characteristics of the SPFH protein superfamily. The transcript levels suggest a coordinated expression of ZsPHB1 and ZsPHB2 genes, while ZsPHB1 was detected in soluble protein extracts depending on the stage of development. Histological examination showed ZsPHB1 is present in all larval tissues, with an intense fluorescence signal observed at the gut. These results suggest a physiologically important role of PHB during Z. subfasciatus development and show its regulation occurs at the transcriptional and post-transcriptional levels. This is the first characterization of PHB in Z. subfasciatus.

Gene Sequences of Potential Targets of Insecticidal PF2 Lectin Identified from the Larval De Novo Transcriptome of the Mexican Bean Weevil (Zabrotes Subfasciatus; Boheman 1833)

Simple Summary

The Mexican bean weevil Zabrotes subfasciatus is a major insect pest of stored beans. We have previously reported that the PF2 lectin, which is a protein found in the desert wild legume Olneya tesota (Palo Fierro), is toxic to Z. subfasciatus by inhibiting its early larval development. The use of proteomic means allowed us to identify PF2 targets in the midgut of Z. subfasciatus larvae. However, efforts to completely elucidate the insecticidal mechanism of PF2, as well as novel potential targets for insecticidal compounds, have been hindered by the lack of available genomic and proteomic information of non-model insects. Therefore, in this work we massively sequenced and analyzed the transcripts expressed in the larval stage of Z. subfasciatus, which is the first transcriptome reported for this insect. A total of 29,029 transcript sequences were identified, of which 30 sequences encode putative targets of PF2. The functional characteristics and biochemical, biological, or molecular roles for 15,124 sequences were established by means of bioinformatics tools. This study significantly increased the available genetic resources for Zabrotes and related insect species and will be helpful for any kind of future study that requires information on genes or protein sequences.

Abstract

The available genomic and proteomic information of non-model organisms is often underrepresented in public databases hindering their study at molecular, cellular, and physiological levels. Information on Zabrotes subfasciatus (Mexican bean weevil) is poorly represented in databases, yet it is a major pest of common beans. We report the transcriptome of Z. subfasciatus larvae; transcripts were sequenced using an Illumina RNA-Seq technology and assembled de novo identifying 29,029 unigenes with an average size of 1168 bp and an N50 value of 2196 bp. About 15,124 unigenes (52%) were functionally annotated and categorized. Further analysis revealed 30 unigene sequences encoding putative targets of the insecticidal PF2 lectin. The complete deduced amino acid sequences of eight selected proteins potentially related to insecticidal mechanism of Palo Fierro 2 (PF2) were used for predicting probable N-glycosylation sites and analyzing phylogenetic relationships with insect sequences. This work provides a dramatic increase in the genetic resources available for Coleopterans and set the basis for developing future studies on biological aspects and potential control strategies for Z. subfasciatus.

The Amylases of Insects

Alpha-amylases are major digestive enzymes that act in the first step of maltopolysaccharide digestion. In insects, these enzymes have long been studied for applied as well as purely scientific purposes. In many species, amylases are produced by multiple gene copies. Rare species are devoid of Amy gene. They are predominantly secreted in the midgut but salivary expression is also frequent, with extraoral activity. Enzymological parameters are quite variable among insects, with visible trends according to phylogeny: Coleopteran amylases have acidic optimum activity, whereas dipteran amylases have neutral preference and lepidopteran ones have clear alkaline preference. The enzyme structure shows interesting variations shaped by evolutionary convergences, such as the recurrent loss of a loop involved in substrate handling. Many works have focused on the action of plant amylase inhibitors on pest insect amylases, in the frame of crop protection by transgenesis. It appears that sensitivity or resistance to inhibitors is finely tuned and very specific and that amylases and their inhibitors have coevolved. The multicopy feature of insect amylases appears to allow tissue-specific or stage-specific regulation, but also to broaden enzymological abilities, such as pH range, and to overcome plant inhibitory defenses.

Genomic and functional characterization of coleopteran insect-specific α-amylase inhibitor gene from Amaranthus species

The smallest 32 amino acid α-amylase inhibitor from Amaranthus hypochondriacus (AAI) is reported. The complete gene of pre-protein (AhAI) encoding a 26 amino acid (aa) signal peptide followed by the 43 aa region and the previously identified 32 aa peptide was cloned successfully. Three cysteine residues and one disulfide bond conserved within known α-amylase inhibitors were present in AhAI. Identical genomic and open reading frame was found to be present in close relatives of A. hypochondriacus namely Amaranthus paniculatus, Achyranthes aspera and Celosia argentea. Interestingly, the 3'UTR of AhAI varied in these species. The highest expression of AhAI was observed in A. hypochondriacus inflorescence; however, it was not detected in the seed. We hypothesized that the inhibitor expressed in leaves and inflorescence might be transported to the seeds. Sub-cellular localization studies clearly indicated the involvement of AhAI signal peptide in extracellular secretion. Full length rAhAI showed differential inhibition against α-amylases from human, insects, fungi and bacteria. Particularly, α-amylases from Helicoverpa armigera (Lepidoptera) were not inhibited by AhAI while Tribolium castaneum and Callosobruchus chinensis (Coleoptera) α-amylases were completely inhibited. Molecular docking of AhAI revealed tighter interactions with active site residues of T. castaneum α-amylase compared to C. chinensis α-amylase, which could be the rationale behind the disparity in their IC₅₀. Normal growth, development and adult emergence of C. chinensis were hampered after feeding on rAhAI. Altogether, the ability of AhAI to affect the growth of C. chinensis demonstrated its potential as an efficient bio-control agent, especially against stored grain pests.