Latsavongsakda Sethaphong

Tertiary model of a plant cellulose synthase

A 3D atomistic model of a plant cellulose synthase (CESA) has remained elusive despite over forty years of experimental effort. Here, we report a computationally predicted 3D structure of 506 amino acids of cotton CESA within the cytosolic region. Comparison of the predicted plant CESA structure with the solved structure of a bacterial cellulose-synthesizing protein validates the overall fold of the modeled glycosyltransferase (GT) domain. The coaligned plant and bacterial GT domains share a six-stranded β-sheet, five α-helices, and conserved motifs similar to those required for catalysis in other GT-2 glycosyltransferases. Extending beyond the cross-kingdom similarities related to cellulose polymerization, the predicted structure of cotton CESA reveals that plant-specific modules (plant-conserved region and class-specific region) fold into distinct subdomains on the periphery of the catalytic region. Computational results support the importance of the plant-conserved region and/or class-specific region in CESA oligomerization to form the multimeric cellulose–synthesis complexes that are characteristic of plants. Relatively high sequence conservation between plant CESAs allowed mapping of known mutations and two previously undescribed mutations that perturb cellulose synthesis in Arabidopsis thaliana to their analogous positions in the modeled structure. Most of these mutation sites are near the predicted catalytic region, and the confluence of other mutation sites supports the existence of previously undefined functional nodes within the catalytic core of CESA. Overall, the predicted tertiary structure provides a platform for the biochemical engineering of plant CESAs.

Interactions of Cations with RNA Loop-Loop Complexes

RNA loop-loop interactions are essential in many biological processes, including initiation of RNA folding into complex tertiary shapes, promotion of dimerization, and viral replication. In this article, we examine interactions of metal ions with five RNA loop-loop complexes of unique biological significance using explicit-solvent molecular-dynamics simulations. These simulations revealed the presence of solvent-accessible tunnels through the major groove of loop-loop interactions that attract and retain cations. Ion dynamics inside these loop-loop complexes were distinctly different from the dynamics of the counterion cloud surrounding RNA and depend on the number of basepairs between loops, purine sequence symmetry, and presence of unpaired nucleotides. The cationic uptake by kissing loops depends on the number of basepairs between loops. It is interesting that loop-loop complexes with similar functionality showed similarities in cation dynamics despite differences in sequence and loop size.

Computational and genetic evidence that different structural conformations of a non-catalytic region affect the function of plant cellulose synthase

Summary Computational modelling of peptide structure, genetic complementation in Arabidopsis thaliana, and confocal microscopy provide evidence that a region between two transmembrane helices may adopt two predominant structural conformations that affect the function of plant cellulose synthase.

Prediction of the structures of the plant-specific regions of vascular plant cellulose synthases and correlated functional analysis

Seed plants express cellulose synthase (CESA) protein isoforms with non-redundant functions, but how the isoforms function differently is unknown. Compared to bacterial cellulose synthases, CESAs have two insertions in the large cytosolic loop: the relatively well-conserved Plant Conserved Region (P-CR) and a Class Specific Region (CSR) that varies between CESAs. Absent any atomic structure of a plant CESA, we used ab initio protein structure prediction and molecular modeling to explore how these plant-specific regions may modulate CESA function. We modeled P-CR and CSR peptides from Arabidopsis thaliana CESAs representing the six clades of seed plant CESAs. As expected, the predicted wild type P-CR structures were similar. Modeling of the mutant P-CR of Atcesa8R362K (fra6) suggested that changes in local structural stability and surface electrostatics may cause the mutant phenotype. Among CSRs within CESAs required for primary wall cellulose synthesis, the amino sequence and the modeled arrangement of helices was most similar in AtCESA1 and AtCESA3. Genetic complementation of known Arabidopsis mutants showed that the CSRs of AtCESA1 and AtCESA3 can function interchangeably in vivo. Analysis of protein surface electrostatics led to ideas about how the surface charges on CSRs may mediate protein–protein interactions. Refined modeling of the P-CR and CSR regions of GhCESA1 from cotton modified their tertiary structures, spatial relationships to the catalytic domain, and preliminary predictions about CESA oligomer formation. Cumulatively, the results provide structural clues about the function of plant-specific regions of CESA.

X3DBio2: A visual analysis tool for biomolecular structure comparison

A major problem in structural biology is the recognition of differences and similarities between related three dimensional (3D) biomolecular structures. Investigating these structure relationships is important not only for understanding of functional properties of biologically significant molecules, but also for development of new and improved materials based on naturally-occurring molecules. We developed a new visual analysis tool, X3DBio2, for 3D biomolecular structure comparison and analysis. The tool is designed for elucidation of structural effects of mutations in proteins and nucleic acids and for assessment of time dependent trajectories from molecular dynamics simulations. X3DBio2 is a freely downloadable open source software and provides tightly integrated features to perform many standard analysis and visual exploration tasks. We expect this tool can be applied to solve a variety of biological problems and illustrate the use of the tool on the example study of the differences and similarities between two proteins of the glycosyltransferase family 2 that synthesize polysaccharides oligomers. The size and conformational distances and retained core structural similarity of proteins SpsA to K4CP represent significant epochs in the evolution of inverting glycosyltransferases.

Nucleic Acid Helical Conformation and Sequence Effects on Cationic Binding

Conformation dependent molecular recognition has often been more associated with proteins which must be able to sense thousands of different molecules within a cell. Despite lacking as extensive a repertoire, nucleic acids also depend on nuances of structure with their environment to gain specificity for regulating genetic duplication, editing, expression, and suppression. In order to explore this topic further, molecular dynamics simulations of nucleic acid dupelexes of DNA and RNA were performed to examine subtleties with their inherent cation binding behavior. We discovered that despite small differences between chemical moieties of DNA and RNA, stark contrasts in counterion interactions occured. In the presence of either 0.1M Na+ or 0.1M K+, ion dynamics were not significantly altered for a given duplex. However, greater deviations were seen between duplexes of different helical forms. A final inquiry into the role of geometry leads us to conclude that helical geometry is responsible for a greater majority of cationic interactions and diffusive ion binding than can be explained exclusively by electrostatics resulting from chemical moieties alone.

Molecular Dynamics and Distribution of Ions in Kissing Loop

RNAs have hierarchical folding of structure which is endowed with abilities to catalyze biochemical reactions, support ligand binding, and proteins recognition. Ionic environment assist RNA to form stable higher order structures. In this study, molecular dynamics simulations were used to analyze the monovalent cationic distributions within RNA loop-loop complexes taken from separate viral species. We demonstrate that cations in show strong preferential distribution around kissing loop region however, ion dynamics do not indicate concrete evidence of specific binding. Cationic spatial localization was observed in a variety of kissing loops. Simulations results reveal the presence of electronegative channels that formed through the major groove of all RNA loop-loop helices and attract and retain the cations. Significant drop of diffusion coefficients was observed for ions inside ionic channels. Effect of sequence on the ion distribution was observed by carrying out mutational studies on the bacterial and viral kissing loops. Molecular dynamics results show strong correlation of ionic propensity regulated by sequence.