Conformation of Systemin, a Polypeptide Activator of Proteinase Inhibitor Synthesis in Plants

Plant Polypeptide Hormone Systemin Prefers Polyproline II Conformation in Solution

Systemin, an 18 amino-acid-signaling peptide, was the first plant polypeptide hormone to be discovered. Earlier structural studies involving NMR spectroscopy indicated a lack of definite structure in solution while circular dichroism spectroscopy suggested the presence of left-handed polyproline II (PPII) conformation. Here, we report the results of molecular dynamics simulations of the peptide in explicit solvent with two different force fields, namely, ff99SBildn and ff99IDPs, both of which showed a large propensity for PPII-like conformations in spite of showing differing features for other conformational characteristics. More remarkably, the conformations with predicted chemical shifts that agreed better with the NMR observations had a larger than average PPII content, especially for the ff99IDPs force field. An independent docking calculation of the molecule with the putative receptor SR160 also retained this conformational preference for PPII structure. The results suggest PPII to be an important class of conformation for systemin which may have a role in its bioactivity.

Effect of Inactivating Mutations on Peptide Conformational Ensembles: The Plant Polypeptide Hormone Systemin

As part of their basal immune mechanism against insect/herbivore attacks, plants have evolved systemic response mechanisms. Such a systemic wound response in tomato was found to involve an 18 amino acid polypeptide called systemin, the first polypeptide hormone to be discovered in plants. Systematic alanine scanning and deletion studies showed differential modulation in its activity, particularly a major loss of function due to alanine substitution at positions 13 and 17 and less extentive loss of function due to substitution at position 12. We have studied the conformational ensembles of wild-type systemin along with its 17 variants by carrying out a total of 5.76 μs of replica-exchange molecular dynamics simulation in an implicit solvent environment. In our simulations, wild-type systemin showed a lack of α-helical and β-sheet structures, in conformity with earlier circular dichroism and NMR data. On the other hand, two regions containing diproline segments showed a tendency to adopt polyproline II structures. Examination of conformational ensembles of the 17 variants revealed a change in the population distributions, suggesting a less flexible structure for alanine substitutions at positions 12 and 13 but not for position 17. Combined with the experimental observations that positions 1-14 of systemin are important for the formation of the peptide-receptor complex, this leads to the hypothesis that loss of conformational flexibility may play a role in the loss of activity of systemin due to the P12A and P13A substitutions, while T17A deactivation probably occurs for a different reason, most likely the loss of the threonine phosphorylation site. We also indicate possible structural reasons why the substitution of the prolines at positions 12 and 13 leads to a loss of conformational freedom in the peptide.

Glycosides of hydroxyproline: some recent, unusual discoveries

Glycosides of hydroxyproline (Hyp) in the plant cell wall matrix were discovered by Lamport and co-workers in the 1960s. Since then, much has been learned about these Hyp-rich glycoproteins. The intent of this review was to compare and contrast some less common structural motifs, in nontraditional roles, to uncover themes. Arabinosylation of short-peptide plant hormones is essential for growth, cell differentiation and defense. In a very recent development, prolyl hydroxylase and arabinosyltransferase activity has been shown to have a direct impact on the growth of root hairs in Arabidopsis thaliana. Pollen allergens of mugwort and ragweed contain proline-rich domains that are hydroxylated and glycosylated and play a structural role. In the case of mugwort, this domain also presents a significant immunogenic epitope. Major crops, including tobacco and maize, have been used to express and produce recombinant proteins of mammalian origin. The risks of plant-imposed glycosylation are discussed. In unicellular eukaryotes, Skp1 (a subunit of the E3(SCF) ubiquitin ligase complex) harbors a key Hyp residue that is modified by a linear pentasaccharide. These modifications may be involved in sensing oxygen levels. A few studies have probed the impact of glycosylation on the structure of Hyp-containing peptides. These have necessarily looked at small, synthetic molecules, since natural peptides and proteins are often isolable in only minuscule amounts and/or are heterogeneous in nature. The characterization of native structural motifs, together with the determination of glycopeptide conformation and properties, holds the key to rationalizing nature's architectural design.

G-quadruplex DNA aptamers generated for systemin

Ligands specific to bioactive molecules play important roles in biomedical researches and applications, such as biological assay, diagnosis and therapy. Systemin is a peptide hormone firstly identified in plant. In this paper we report the selection of a group of DNA aptamers that can specifically bind to systemin. Through comparing the predicted secondary structures of all the aptamers, a hairpin structure with G-rich loop was determined to be the binding motif of these aptamers. The G-rich loop region of this binding motif was further characterized to fold into an antiparallel G-quadruplex by truncation-mutation assay and CD spectrum. The apparent equilibrium dissociation constant (K(d)) of one strong binding sequence (S-5-1) was measured to be 0.5 μM. The specificity assay shows that S-5-1 strongly bind to whole systemin, weakly bind to truncated or mutated systemin and does not bind to the scrambled peptide with the same amino acid composition as systemin. The high affinity and specificity make S-5-1 hold potentials to serve as a molecular ligand applied in detection, separation and functional investigation of systemin in plants.

Recent advances in capillary electrophoresis of peptides

The article gives a comprehensive review on the recent developments in the applications of high-performance capillary electromigration methods, including zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography and electrochromatography, to analysis, preparation and physicochemical characterization of peptides. The article presents new approaches to the theoretical description and experimental verification of electromigration behavior of peptides, and covers the methodological aspects of capillary electroseparations of peptides, such as strategy and rules for the rational selection of separation mode and experimental conditions, sample treatment, suppression of peptide adsorption to the inner capillary wall, new developments in individual separation modes and new designs of detection systems. Several types of applications of capillary electromigration methods to peptide analysis are presented: conventional qualitative and quantitative analysis for determination of purity, determination in biomatrices, monitoring of physical and chemical changes and enzymatic conversions, amino acid and sequence analysis and peptide mapping of proteins. Some examples of micropreparative peptide separations are given and capabilities of capillary electromigration techniques to provide important physicochemical characteristics of peptides are demonstrated.