Pectin Digestion in Herbivorous Beetles: Impact of Pseudoenzymes Exceeds That of Their Active Counterparts.

Roy Kirsch,Yannick Pauchet,Grit Kunert,Heiko Vogel

doi:10.3389/fphys.2019.00685

Roy Kirsch, Yannick Pauchet + Show 2 more

Open Access

https://doi.org/10.3389/fphys.2019.00685

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Abstract

Many protein families harbor pseudoenzymes that have lost the catalytic function of their enzymatically active counterparts. Assigning alternative function and importance to these proteins is challenging. Because the evolution toward pseudoenzymes is driven by gene duplication, they often accumulate in multigene families. Plant cell wall-degrading enzymes (PCWDEs) are prominent examples of expanded gene families. The pectolytic glycoside hydrolase family 28 (GH28) allows herbivorous insects to break down the PCW polysaccharide pectin. GH28 in the Phytophaga clade of beetles contains many active enzymes but also many inactive counterparts. Using functional characterization, gene silencing, global transcriptome analyses, and recordings of life history traits, we found that not only catalytically active but also inactive GH28 proteins are part of the same pectin-digesting pathway. The robustness and plasticity of this pathway and thus its importance for the beetle is supported by extremely high steady-state expression levels and counter-regulatory mechanisms. Unexpectedly, the impact of pseudoenzymes on the pectin-digesting pathway in Phytophaga beetles exceeds even the influence of their active counterparts, such as a lowered efficiency of food-to-energy conversion and a prolongation of the developmental period.

Highlights

Though plants contain all the nutrients herbivorous insects need, dependence on plants as a food source is challenging for two reasons: nutrient amounts and ratios are highly variable, and nutrient requirements are not uniform over an insect’s life cycle (Schoonhoven et al, 2005)
The main storage polysaccharide in plants, is of great importance as the source of energy, which fuels insect growth and development. This polysaccharide can be digested by amylases, Pectin Digestion in Herbivorous Beetles which are widespread in herbivorous insects (Kaur et al, 2014)
This carbohydrate network can be broken down by plant cell wall-degrading enzymes (PCWDEs), which mainly belong to different families of carbohydrate esterases, polysaccharide lyases and glycoside hydrolases (Gilbert, 2010)

Summary

INTRODUCTION

Though plants contain all the nutrients herbivorous insects need, dependence on plants as a food source is challenging for two reasons: nutrient amounts and ratios are highly variable, and nutrient requirements are not uniform over an insect’s life cycle (Schoonhoven et al, 2005). PGs persisted after the initial HGT early in the evolution of “Phytophaga” beetles, and their genes have duplicated and are under continued action of purifying selection while the corresponding proteins have functionally diversified These facts strongly indicate that an important function of PGs is to promote herbivory in this clade of beetles, which represents about 50% of all herbivorous insects (Strong et al, 1984). We simultaneously silenced the three endoPGs of the mustard leaf beetle Phaedon cochleariae, and, in another RNAi treatment, three GH28 family members that had most likely lost their PG enzymatic activity (Kirsch et al, 2014) This simultaneous knockdown, enabled us to test whether inactive GH28 family members continue to play a role in pectin hydrolysis, their ancestral function, even if they are not hydrolyzing polygalacturonan. Gene silencing allows us to study the significance of active enzymes and their pseudoenzyme counterparts and to understand why they became inactive toward a substrate during evolution while still under strong purifying selection (Kirsch et al, 2014)

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DISCUSSION