A model for the interaction of wheat monomeric and dimeric protein inhibitors with alpha-amylase

Genetic architecture underlying the expression of eight α-amylase trypsin inhibitors

KEY MESSAGE

Wheat cultivars largely differ in the content and composition of ATI proteins, but heritability was quite low for six out of eight ATIs. The genetic architecture of ATI proteins is built up of few major and numerous small effect QTL. Amylase trypsin inhibitors (ATIs) are important allergens in baker's asthma and suspected triggers of non-celiac wheat sensitivity (NCWS) inducing intestinal and extra-intestinal inflammation. As studies on the expression and genetic architecture of ATI proteins in wheat are lacking, we evaluated 149 European old and modern bread wheat cultivars grown at three different field locations for their content of eight ATI proteins. Large differences in the content and composition of ATIs in the different cultivars were identified ranging from 3.76 pmol for ATI CM2 to 80.4 pmol for ATI 0.19, with up to 2.5-fold variation in CM-type and up to sixfold variation in mono/dimeric ATIs. Generally, heritability estimates were low except for ATI 0.28 and ATI CM2. ATI protein content showed a low correlation with quality traits commonly analyzed in wheat breeding. Similarly, no trends were found regarding ATI content in wheat cultivars originating from numerous countries and decades of breeding history. Genome-wide association mapping revealed a complex genetic architecture built of many small, few medium and two major quantitative trait loci (QTL). The major QTL were located on chromosomes 3B for ATI 0.19-like and 6B for ATI 0.28, explaining 70.6 and 68.7% of the genotypic variance, respectively. Within close physical proximity to the medium and major QTL, we identified eight potential candidate genes on the wheat reference genome encoding structurally related lipid transfer proteins. Consequently, selection and breeding of wheat cultivars with low ATI protein amounts appear difficult requiring other strategies to reduce ATI content in wheat products.

The impact of single nucleotide polymorphism in monomeric alpha-amylase inhibitor genes from wild emmer wheat, primarily from Israel and Golan

Background

Various enzyme inhibitors act on key insect gut digestive hydrolases, including alpha-amylases and proteinases. Alpha-amylase inhibitors have been widely investigated for their possible use in strengthening a plant's defense against insects that are highly dependent on starch as an energy source. We attempted to unravel the diversity of monomeric alpha-amylase inhibitor genes of Israeli and Golan Heights' wild emmer wheat with different ecological factors (e.g., geography, water, and temperature). Population methods that analyze the nature and frequency of allele diversity within a species and the codon analysis method (comparing patterns of synonymous and non-synonymous changes in protein coding sequences) were used to detect natural selection.

Results

Three hundred and forty-eight sequences encoding monomeric alpha-amylase inhibitors (WMAI) were obtained from 14 populations of wild emmer wheat. The frequency of SNPs in WMAI genes was 1 out of 16.3 bases, where 28 SNPs were detected in the coding sequence. The results of purifying and the positive selection hypothesis (p < 0.05) showed that the sequences of WMAI were contributed by both natural selection and co-evolution, which ensured conservation of protein function and inhibition against diverse insect amylases. The majority of amino acid substitutions occurred at the C-terminal (positive selection domain), which ensured the stability of WMAI. SNPs in this gene could be classified into several categories associated with water, temperature, and geographic factors, respectively.

Conclusions

Great diversity at the WMAI locus, both between and within populations, was detected in the populations of wild emmer wheat. It was revealed that WMAI were naturally selected for across populations by a ratio of dN/dS as expected. Ecological factors, singly or in combination, explained a significant proportion of the variations in the SNPs. A sharp genetic divergence over very short geographic distances compared to a small genetic divergence between large geographic distances also suggested that the SNPs were subjected to natural selection, and ecological factors had an important evolutionary role in polymorphisms at this locus. According to population and codon analysis, these results suggested that monomeric alpha-amylase inhibitors are adaptively selected under different environmental conditions.

Wheat and barley allergens associated with baker's asthma. Glycosylated subunits of the alpha-amylase-inhibitor family have enhanced IgE-binding capacity

A 16 kDa protein, designated CM16*, which strongly binds IgE from baker's-asthma patients has been identified as a glycosylated form of the previously reported WTAI-CM16, which is a subunit of the wheat tetrameric alpha-amylase inhibitor. A glycosylated form (CMb*) of BTAI-CMb, the equivalent inhibitor subunit from barley, has been also found to have significantly enhanced IgE-binding capacity. In all, 14 purified members of the alpha-amylase/trypsin-inhibitor family showed very different IgE-binding capacities when tested by a dot-blot assay. The glycosylated components CM16*, CMb* and the previously described non-glycosylated 14.5 kDa allergen from barley (renamed BMAI-1) were found to be the strongest allergens.

Genes encoding α-amylase inhibitors are located in the short arms of chromosomes 3B, 3D and 6D of wheat (Triticum aestivum L.)

Three α-amylase inhibitors, designated Inh. I, II and III have been purified from the 70% ethanol extract of hexaploid wheat (Triticum aestivum L.) and characterized by amino acid analysis, N-terminal amino acid sequencing and enzyme inhibition tests. Inhibitors I and III have identical N-terminal sequences and inhibitory properties to those of the previously described 0.19/0.53 group of dimeric inhibitors. Inhibitor II has an N-terminal sequence which is identical to that of the previously described 0.28 monomeric inhibitor, but differs from it in that in addition to being active against α-amylase from Tenebrio molitor, it is also active against mammalian salivary and pancreatic α-amylases. Compensating nulli-tetrasomic and ditelosomic lines of wheat cv. 'Chinese Spring' have been analysed by two-dimensional electrophoresis, under conditions in which there is no overlap of the inhibitors with other proteins, and the chromosomal locations of the genes encoding these inhibitors have been established: genes for Inh. I and Inh. III are in the short arms of chromosomes 3B and 3D, respectively, and that for Inh. II in the short arm of chromosome 6D.

The purification of a novel amylase from Bacillus subtilis and its inhibition by wheat proteins

A new alpha-amylase (EC 3.2.1.1) from Bacillus subtilis was purified by affinity chromatography. The molecular weight of the purified enzyme, estimated from sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, was 93000, which is very different from the molecular weights of two well-characterized amylases from B. subtilis. Electrofocusing showed an isoelectric point of 5. Amylase shows a broad maximum of activity between pH 6 and 7; maximal inhibition of enzyme by wheat-protein alpha-amylase inhibitors is displayed at pH 7.

Isolation and characterization of four inhibitors from wheat flour which display differential inhibition specificities for human salivary and human pancreatic alpha-amylases

Four alpha-amylase (1,4-alpha-D-glucan glucanohydrolase, EC 3.2.1.1) inhibitors were isolated from an albumin fraction of wheat flour by ion-exchange and gel-filtration chromatography. The purified inhibitors were characterized according to their electrophoretic mobilities, molecular weights, carbohydrate, content, sulphydryl content, susceptibility to proteolytic digestion and specificities in inhibiting human salivary and pancreatic alpha-amylases. The properties of these inhibitors ae compared to similar proteins isolated by other workers.

Interaction of human alpha-amylases with inhibitors from wheat flour

The interaction of four purified alpha-amylase (1,4-alpha-D-glucan glucanohydrolase, EC 3.2.1.1) inhibitors with human salivary and pancreatic alpha-amylases was investigated. The inhibitory activity of the four proteins towards salivary alpha-amylase was significantly increased by pre-incubation of the enzyme with inhibitor before adding substrate. This effect was not observed with the inhibition of pancreatic alpha-amylase by inhibitors 1 and 2. Inhibition of both amylases was affected to different degrees by incubating starch with inhibitor prior to the addition of enzyme. Maltose, at concentrations which only slightly affected amylase activity, prevented the inhibition of both enzymes by all four inhibitors. Gel filtration studies on salivary amylase-inhibitor mixtures showed the formation of EI complexes on a mol-to-mol ratio. A similar complex between pancreatic alpha-amylase and inhibitor 4 was observed, though complex formation between pancreatic alpha-amylase and the other inhibitors was not clearly demonstrated.

Interaction of wheat monomeric and dimeric protein inhibitors with alpha-amylase from yellow mealworm (Tenebrio molitor L. larva)

The highly purified alpha-amylase from Tenebrio molitor L. larva (yellow mealworm) reversibly combines with two closely related homogeneous glycoprotein inhibitors, one dimeric (termed 'inhibitor 0.19') and one monomeric (termed 'inhibitor 0.28'), from wheat flour. As established by means of difference spectroscopy and kinetic studies, molar combining ratios for the amylase--inhibitor-0.19 and amylase-inhibitor-0.28 complexes were 1:1 and 1:2 respectively. Two amylase--inhibitor-0.19 complexes with slightly different retention volumes on Bio-Gel P-300 and only one amylase--inhibitor-0.28 complex were observed. Dissociation constants of the amylase--inhibitor-0.19 and amylase--inhibitor-0.28 complexes were 0.85 nM and 0.13 nM respectively. A strong tendency of both complexes to precipitate under an ultracentrifugal field was observed; the minimum molecular weight calculated for the two complexes under such conditions was approx. 95 000. The two complexes showed difference spectra indicating involvement of structurally related or identical tryptophyl side chains in the binding of inhibitors 0.28 and 0.19 to the amylase. A model summarizing the main features of the inhibition of the insect amylase by the two wheat protein inhibitors is proposed.