Tailoring the specificity of a plant cystatin toward herbivorous insect digestive cysteine proteases by single mutations at positively selected amino acid sites

Deleterious effects of plant cystatins against the banana weevil Cosmopolites sordidus

The general potential of plant cystatins for the development of insect-resistant transgenic plants still remains to be established given the natural ability of several insects to compensate for the loss of digestive cysteine protease activities. Here we assessed the potential of cystatins for the development of banana lines resistant to the banana weevil Cosmopolites sordidus, a major pest of banana and plantain in Africa. Protease inhibitory assays were conducted with protein and methylcoumarin (MCA) peptide substrates to measure the inhibitory efficiency of different cystatins in vitro, followed by a diet assay with cystatin-infiltrated banana stem disks to monitor the impact of two plant cystatins, oryzacystatin I (OC-I, or OsCYS1) and papaya cystatin (CpCYS1), on the overall growth rate of weevil larvae. As observed earlier for other Coleoptera, banana weevils produce a variety of proteases for dietary protein digestion, including in particular Z-Phe-Arg-MCA-hydrolyzing (cathepsin L-like) and Z-Arg-Arg-MCA-hydrolyzing (cathepsin B-like) proteases active in mildly acidic conditions. Both enzyme populations were sensitive to the cysteine protease inhibitor E-64 and to different plant cystatins including OsCYS1. In line with the broad inhibitory effects of cystatins, OsCYS1 and CpCYS1 caused an important growth delay in young larvae developing for 10 days in cystatin-infiltrated banana stem disks. These promising results, which illustrate the susceptibility of C. sordidus to plant cystatins, are discussed in the light of recent hypotheses suggesting a key role for cathepsin B-like enzymes as a determinant for resistance or susceptibility to plant cystatins in Coleoptera.

Developmentally linked changes in proteases and protease inhibitors suggest a role for potato multicystatin in regulating protein content of potato tubers

The soluble protein fraction of fully developed potato (Solanum tuberosum L.) tubers is dominated by patatin, a 40 kD storage glycoprotein, and protease inhibitors. Potato multicystatin (PMC) is a multidomain Cys-type protease inhibitor. PMC effectively inhibits degradation of patatin by tuber proteases in vitro. Herein we show that changes in PMC, patatin concentration, activities of various proteases, and their gene expression are temporally linked during tuber development, providing evidence that PMC has a role in regulating tuber protein content in vivo. PMC was barely detectable in non-tuberized stolons. PMC transcript levels increased progressively during tuberization, concomitant with a 40-fold increase in PMC concentration (protein basis) as tubers developed to 10 g fresh wt. Further increases in PMC were comparatively modest (3.7-fold) as tubers developed to full maturity (250 g). Protease activity declined precipitously as PMC levels increased during tuberization. Proteolytic activity was highest in non-tuberized stolons and fell substantially through the 10-g fresh wt stage. Cys-type proteases dominated the pre-tuberization and earliest stages of tuber development. Increases in patatin transcript levels during tuberization were accompanied by a notable lag in patatin accumulation. Patatin did not begin to accumulate substantially on a protein basis until tubers had reached the 10-g stage, wherein protease activity had been inhibited by approximately 60%. These results indicate that a threshold level of PMC (ca. 3 microg tuber(-1), 144 ng mg(-1) protein) is needed to favor patatin accumulation. Collectively, these results are consistent with a role for PMC in facilitating the accumulation of proteins in developing tubers by inhibiting Cys-type proteases.

Nucleocytoplasmic transit of human alpha1-antichymotrypsin in tobacco leaf epidermal cells

Recently, we have observed a nuclear localization for human alpha(1)-antichymotrypsin (AACT) expressed in the cytosol of transgenic Bright Yellow-2 (BY-2) tobacco cultured cells (see accompanying paper: Benchabane, M., Saint-Jore-Dupas, C., Bardor, M., Faye, L., Michaud, D. and Gomord, V. (2008a) Targeting and post-translational processing of human alpha(1)-antichymotrypsin in BY-2 tobacco cultured cells. Plant Biotechnol. J. doi: 10.1111/j.1467-7652.2008.00382.x). In the present article, we assess whether the intrinsic DNA-binding activity of AACT can explain its nuclear localization, and whether this same activity has an impact on its protease inhibitory potency and stability in planta. An engineered form of AACT with no DNA-binding activity, rAACTDeltaK, was compared with the wild-type polypeptide, rAACT, in terms of chymotrypsin inhibitory potency, stability in planta and distribution in tobacco cells. In accordance with available data reporting distinct sites for protease inhibition and DNA binding, rAACT and rAACTDeltaK showed similar antichymotrypsin activity, similar to the activity of native AACT purified from human plasma. As observed for AACT in BY-2 tobacco cells, a green fluorescent protein (GFP)-AACT fusion transiently expressed in the cytosol of tobacco leaf epidermal cells was detected mainly in the nucleus by confocal laser microscopy. By contrast, rAACTDeltaK expressed as a GFP fusion showed a balanced distribution between the cytosol and the nucleus, similar to the distribution pattern of free GFP exhibiting no DNA-binding affinity. In line with immunodetection data showing higher accumulation levels for GFP-AACT in tobacco leaf cells, rAACTDeltaK was more susceptible than rAACT to tryptic digestion in the presence of DNA. Overall, these observations suggest the following: (i) a retention effect of DNA on AACT in the nucleus; and (ii) a stabilizing effect of the AACT-DNA interaction on rAACT challenged with non-target proteases, which, possibly, may be useful in protecting this protein in plant expression platforms.

Viral cystatin evolution and three-dimensional structure modelling: a case of directional selection acting on a viral protein involved in a host-parasitoid interaction

Background

In pathogens, certain genes encoding proteins that directly interact with host defences coevolve with their host and are subject to positive selection. In the lepidopteran host-wasp parasitoid system, one of the most original strategies developed by the wasps to defeat host defences is the injection of a symbiotic polydnavirus at the same time as the wasp eggs. The virus is essential for wasp parasitism success since viral gene expression alters the immune system and development of the host. As a wasp mutualist symbiont, the virus is expected to exhibit a reduction in genome complexity and evolve under wasp phyletic constraints. However, as a lepidopteran host pathogenic symbiont, the virus is likely undergoing strong selective pressures for the acquisition of new functions by gene acquisition or duplication. To understand the constraints imposed by this particular system on virus evolution, we studied a polydnavirus gene family encoding cyteine protease inhibitors of the cystatin superfamily.

Results

We show that cystatins are the first bracovirus genes proven to be subject to strong positive selection within a host-parasitoid system. A generated three-dimensional model of Cotesia congregata bracovirus cystatin 1 provides a powerful framework to position positively selected residues and reveal that they are concentrated in the vicinity of actives sites which interact with cysteine proteases directly. In addition, phylogenetic analyses reveal two different cystatin forms which evolved under different selective constraints and are characterized by independent adaptive duplication events.

Conclusion

Positive selection acts to maintain cystatin gene duplications and induces directional divergence presumably to ensure the presence of efficient and adapted cystatin forms. Directional selection has acted on key cystatin active sites, suggesting that cystatins coevolve with their host target. We can strongly suggest that cystatins constitute major virulence factors, as was already proposed in previous functional studies.

Preventing unintended proteolysis in plant protein biofactories

Numerous reports have been published over the last decade assessing the potential of plants as useful hosts for the heterologous expression of clinically useful proteins. Significant progress has been made, in particular, in optimizing transgene transcription and translation in plants, and in elucidating the complex post-translational modifications of proteins typical of the plant cell machinery. In this article, we address the important issue of recombinant protein degradation in plant expression platforms, which directly impacts on the final yield, homogeneity and overall quality of the resulting protein product. Unlike several more stable and structurally less complex pharmaceuticals, recombinant proteins present a natural tendency to structural heterogeneity, resulting in part from the inherent instability of polypeptide chains expressed in heterologous environments. Proteolytic processing, notably, may dramatically alter the structural integrity and overall accumulation of recombinant proteins in plant expression systems, both in planta during expression and ex planta after extraction. In this article, we describe the current strategies proposed to minimize protein hydrolysis in plant protein factories, including organ-specific transgene expression, organelle-specific protein targeting, the grafting of stabilizing protein domains to labile proteins, protein secretion in natural fluids and the co-expression of companion protease inhibitors.

Hybrid protease inhibitors for pest and pathogen control--a functional cost for the fusion partners?

Fusion proteins integrating dual pesticidal functions have been devised over the last 10 years to improve the effectiveness and potential durability of pest-resistant transgenic crops, but little attention has been paid to the impact of the fusion partners on the actual activity of the resulting hybrids. Here we assessed the ability of the rice cysteine protease inhibitor, oryzacystatin I (OCI), to retain its protease inhibitory potency when used as a template to devise hybrid inhibitors with dual activity against papain-like proteases and carboxypeptidase A (CPA). C-terminal variants of OCI were generated by fusing to its C-terminal end: (i) the primary inhibitory site of the small CPA inhibitor potato carboxypeptidase inhibitor (PCI, amino acids 35-39); or (ii) the complete sequence of PCI (a.a. 1-39). The hybrid inhibitors were expressed in E. coli and tested for their inhibitory activity against papain, CPA and digestive cysteine proteases of herbivorous and predatory arthropods. In contrast with the primary inhibitory site of PCI, the entire PCI attached to OCI was as active against CPA as free, purified PCI. The OCI-PCI hybrids also showed activity against papain, but the presence of extra amino acids at the C terminus of OCI negatively altered its inhibitory potency against cysteine proteases. This negative effect, although not preventing dual binding to papain and CPA, was correlated with an increased binding affinity for papain presumably due to non-specific interactions with the PCI domain. These results confirm the potential of OCI and PCI for the design of fusion inhibitors with dual protease inhibitory activity, but also point out the possible functional costs associated with protein domain grafting to recipient pesticidal proteins.