Purification and Partial Characterization of Trypsin-Specific Proteinase Inhibitors from Pigeonpea Wild Relative Cajanus platycarpus L. (Fabaceae) Active against Gut Proteases of Lepidopteran Pest Helicoverpa armigera

Development of a rapid process for purification of Bowman-Birk and Kunitz inhibitors from legume seeds, and evaluation of their biophysical, insecticidal, and antimicrobial properties

Bowman-Birk inhibitor (BBI ~10 kDa) and Kunitz inhibitor (KI ~20 kDa) are serine protease/proteinase inhibitor(s) [PI(s)] ubiquitously found in several Leguminous plant species with insecticidal and therapeutic properties. Due to narrow molecular mass differences, the separation of these inhibitors from a single seed variety is tedious. The present study is aimed to develop a rapid protocol (<24 h) for purifying BBI and KI from legume seeds using mild trichloroacetic acid (TCA) extraction followed by trypsin-affinity chromatography. The mature seeds of Vigna radiata and Cajanus platycarpus are used as a model to purify BBI and KI using this protocol. The BBI and KI purified from the seeds of V. radiata are labeled as VrBBI & VrKI, and C. platycarpus are labeled as CpBBI & CpKI, respectively. These PIs are confirmed by immunodetection and MALDI-TOF studies and further characterized for their structural (CD & fluorescence spectroscopy) and functional properties (temperature & DTT stability). BBI(s) purified using the above process are effective in the management of castor semi-looper 'Achaea janata', while KI(s) are effective in the management of pod borer 'Helicoverpa armigera'. Besides, both BBI(s) and KI(s) have significant potential in controlling the growth of methicillin-sensitive 'Staphylococcus aureus', a gram-positive pathogenic bacterium.

Plant Kunitz Inhibitors and Their Interaction with Proteases: Current and Potential Pharmacological Targets

The action of proteases can be controlled by several mechanisms, including regulation through gene expression; post-translational modifications, such as glycosylation; zymogen activation; targeting specific compartments, such as lysosomes and mitochondria; and blocking proteolysis using endogenous inhibitors. Protease inhibitors are important molecules to be explored for the control of proteolytic processes in organisms because of their ability to act on several proteases. In this context, plants synthesize numerous proteins that contribute to protection against attacks by microorganisms (fungi and bacteria) and/or invertebrates (insects and nematodes) through the inhibition of proteases in these organisms. These proteins are widely distributed in the plant kingdom, and are present in higher concentrations in legume seeds (compared to other organs and other botanical families), motivating studies on their inhibitory effects in various organisms, including humans. In most cases, the biological roles of these proteins have been assigned based mostly on their in vitro action, as is the case with enzyme inhibitors. This review highlights the structural evolution, function, and wide variety of effects of plant Kunitz protease inhibitors, and their potential for pharmaceutical application based on their interactions with different proteases.

Response of Midgut Trypsin- and Chymotrypsin-Like Proteases of Helicoverpa armigera Larvae Upon Feeding With Peanut BBI: Biochemical and Biophysical Characterization of PnBBI

Proteinase/Protease inhibitors (PIs) from higher plants play an important role in defense and confer resistance against various insect pests and pathogens. In the present study, Bowman-Birk Inhibitor (BBI) was purified from mature seeds of an interspecific advanced hybrid peanut variety (4368-1) using chromatographic techniques. The biochemical and biophysical characteristics such as low molecular mass, presence of several isoinhibitors and higher-ordered dimer/tetramer, predominance of antiparallel β-sheets and random coils in secondary structure, reactive sites against trypsin and chymotrypsin, broad spectrum of stability toward extreme pH and temperature along with MALDI TOF-TOF analysis (ProteomeXchange identifier PXD016933) ascertained the purified biomolecule from peanut as BBI (PnBBI). Surface plasmon resonance competitive binding analysis revealed the bifunctional PnBBI is a trypsin specific inhibitor with 1:2 stoichiometry as compared to chymotrypsin. A concentration-dependent self-association tendency of PnBBI was further confirmed by 'red shift' in the far-UV CD spectra. Furthermore, the insecticidal potential of PnBBI against Helicoverpa armigera was assessed by in vitro assays and in vivo feeding experiments. A significant reduction in larval body weight was observed with concomitant attenuation in the activity of midgut trypsin-like proteases of H. armigera (HaTPs) fed on PnBBI supplemented diet. The one and two-dimensional zymography studies revealed the disappearance of several isoforms of HaTP upon feeding with PnBBI. qRT-PCR analysis further suggests the role of PnBBI in not only inhibiting the activity of midgut trypsin and chymotrypsin-like proteases but also in modulating their expression. Taken together, the results provide a biochemical and molecular basis for introgressed resistance in peanut interspecific advanced hybrid variety against H. armigera.

Improving Human Dietary Choices Through Understanding of the Tolerance and Toxicity of Pulse Crop Constituents

Chickpea, dry bean, dry pea, and lentil are prominent dietary grain legumes commonly referred to as pulses. Pulses have been a staple component of the human diet for more than 8,000 years; however, in the last 70 years they have virtually disappeared from most Western diets. Reduced intake has occurred concomitantly with inadequate dietary fiber consumption and the onset of the obesity pandemic. Misinformation about tolerance and toxicity of several pulse crop constituents remains a barrier to public health efforts to increase dietary intake. Of particular concern are lectins which participate in agglutination reactions with cell surface proteins and galacto-oligosaccharides which have been associated with intestinal discomfort and flatulence. The scientific basis of these concerns is reviewed.

Comparative transcriptome analysis of pigeonpea, Cajanus cajan (L.) and one of its wild relatives Cajanus platycarpus (Benth.) Maesen

Pigeonpea is a major source of dietary protein to the vegetarian population of the Indian sub-continent. Crop improvement to mitigate biotic and abiotic stresses for realization of its potential yield and bridging yield gap is the need of the hour. Availability of limited genomic resources in the cultivated germplasm, however, is a serious bottleneck towards successful molecular breeding for the development of superior genotypes in pigeonpea. In view of this, improvement of pigeonpea can be attempted through transgenesis or by exploiting genetic resources from its wild relatives. Pigeonpea wild relatives are known to be bestowed with agronomic traits of importance; discovery and deployment of genes from them can provide a lucrative option for crop improvement. Understanding molecular signatures of wild relatives would not only provide information about the mechanism behind desired traits but also enable us to extrapolate the information to cultivated pigeonpea. The present study deals with the characterization of leaf transcriptomes of Cajanus cajan and one of its wild relatives, Cajanus platycarpus. Illumina sequencing revealed 0.11 million transcripts in both the species with an annotation of 0.09 million (82%) transcripts using BLASTX. Comparative transcriptome analyses on the whole, divulged cues about the wild relative being vigilant and agile. Gene ontology and Mapman analysis depicted higher number of transcripts in the wild relative pertaining to signaling, transcription factors and stress responsive genes. Further, networking between the differentially expressed MapMan bins demonstrated conspicuous interactions between different bins through 535 nodes (512 Genes and 23 Pathways) and 1857 edges. The authenticity of RNA-seq analysis was confirmed by qRT-PCR. The information emanating from this study can provide valuable information and resource for future translational research including genome editing to alleviate varied stresses. Further, this learning can be a platform for in-depth investigations to decipher molecular mechanisms for mitigation of various stresses in the wild relative.

Purification and characterization of Bowman-Birk and Kunitz isoinhibitors from the seeds of Rhynchosia sublobata (Schumach.) Meikle, a wild relative of pigeonpea

Rhynchosia sublobata, a wild relative of pigeonpea, possesses defensive proteinase/protease inhibitors (PIs). Characterization of trypsin specific PIs (RsPI) separated from seeds by column chromatography using 2-D gel electrophoresis and Edman degradation method identified R. sublobata possessed both Bowman-Birk isoinhibitors (RsBBI) and Kunitz isoinhibitors (RsKI). A quick method was developed to separate RsBBI and RsKI from RsPI based on their differential solubility in TCA and acetate buffer. N-terminus sequencing of RsBBI and RsKI by MALDI-ISD ascertained the presence of Bowman Birk and Kunitz type isoinhibitors in R. sublobata. RsBBI (9216 Da) and RsKI (19,412 Da) exhibited self-association pattern as revealed by western blotting with anti-BBI antibody and MALDI-TOF peptide mass fingerprint analysis, respectively. RsBBI and RsKI varied significantly in their biochemical, biophysical and insecticidal properties. RsBBI inhibited the activity of trypsin (Ki = 128.5 ± 4.5 nM) and chymotrypsin (Ki = 807.8 ± 23.7 nM) while RsKI (Ki = 172.0 ± 9.2 nM) inhibited the activity of trypsin alone, by non-competitive mode. The trypsin inhibitor (TI) and chymotrypsin inhibitor (CI) activities of RsBBI were stable up to 100 °C. But, RsBBI completely lost its TI and CI activities on reduction with 3 mM DTT. Conversely, RsKI lost its TI activity on heating at 100 °C and retained >60% of its TI activity in presence of 3 mM DTT. CD spectroscopic studies on RsBBI and RsKI showed their secondary structural elements in the following order: random coils > β-sheets/β-turns > α-helix. However, RsKI showed reversible denaturation midpoint (Tm) of 75 °C. Further, the significant inhibitory activity of RsBBI (IC₅₀ = 24 ng) and RsKI (IC₅₀ = 59 ng) against trypsin-like gut proteases of Achaea janata (AjGPs) and Helicoverpa armigera (HaGPs) suggest them as potential biomolecules in the management of A. janata and H. armigera, respectively.

Biochemical properties of a bacterially-expressed Bowman-Birk inhibitor from Rhynchosia sublobata (Schumach.) Meikle seeds and its activity against gut proteases of Achaea janata

Crude proteinase inhibitors (CPIs) extracted from the seeds of Rhynchosia sublobata, a wild relative of pigeon pea showed pronounced inhibitory activity on the larval gut trypsin-like proteases of lepidopteran insect pest - Achaea janata. Consequently, a full-length cDNA of Bowman-Birk inhibitor gene (RsBBI1) was cloned from the immature seeds of R. sublobata. It contained an ORF of 360 bp encoding a 119-amino acid polypeptide (13.3 kDa) chain with an N-terminus signal sequence comprising of 22 amino acids. The amino acid sequence and phylogenetic analysis together revealed that RsBBI1 exhibited a close relation with BBIs from soybean and Phaseolus spp. A cDNA sequence corresponding to RsBBI1 mature protein (89 amino acid stretch) was expressed in E. coli. The recombinant rRsBBI1 protein with a molecular mass of 9.97 kDa was purified using trypsin affinity chromatography. The purified rRsBBI1 exhibited non-competitive mode of inhibition of both bovine trypsin (Ki of 358 ± 11 nM) and chymotrypsin (Ki of 446 ± 9 nM). Its inhibitory activity against these proteases was stable at high temperatures (>95 °C) and a wide pH range but sensitive to reduction with dithiothreitol (DTT), indicating the importance of disulphide bridges in exhibiting its activity. Also, rRsBBI1 showed significant inhibitory activity (IC₅₀ = 70 ng) on A. janata larval gut trypsin-like proteases (AjGPs). Conversely, it showed <1% inhibitory activity (IC₅₀ = 8 μg) on H. armigera larval gut trypsin-like proteases (HaGPs) than it has against AjGPs. Besides, in vivo feeding experiments clearly indicated the deleterious effects of rRsBBI1 on larval growth and development in A. janata which suggests it can be further exploited for such properties.