The role of cell contraction and adhesion in dictyostelium motility.

The crawling motion of Dictyostelium discoideum on substrata involves a number of coordinated events including cell contractions and cell protrusions. The mechanical forces exerted on the substratum during these contractions have recently been quantified using traction force experiments. Based on the results from these experiments, we present a biomechanical model of the contraction phase of Dictyostelium discoideum motility with an emphasis on the adhesive properties of the cell-substratum contact. Our model assumes that the cell contracts at a constant rate and is bound to the substratum by adhesive bridges that are modeled as elastic springs. These bridges are established at a spatially uniform rate while detachment occurs at a spatially varying, load-dependent rate. Using Monte Carlo simulations and assuming a rigid substratum, we find that the cell speed depends only weakly on the detachment kinetics of the cell-substratum interface, in agreement with experimental data. By varying the parameters that control the adhesive and contractile properties of the cell, we are able to make testable predictions. We also extend our model to include a flexible substrate and show that our model is able to produce substratum deformations and force patterns that are quantitatively and qualitatively in agreement with experimental data.

Elastic properties and mechanical stability of chiral and filled viral capsids.

The elasticity and mechanical stability of empty and filled viral capsids under external force loading are studied in a combined analytical and numerical approach. We analyze the influence of capsid structure and chirality on the mechanical properties. We find that generally skew shells have lower stretching energy. For large Föppl-von Kármán numbers gamma (gamma approximately 10(5)), skew structures are stiffer in their elastic response than nonchiral ones. The discrete structure of the capsules not only leads to buckling for large gamma but also influences the breakage behavior of capsules below the buckling threshold: the rupture force shows a gamma1/4 scaling rather than a gamma1/2 scaling as expected from our analytical results for continuous shells. Filled viral capsids are exposed to internal anisotropic pressure distributions arising from regularly packaged DNA coils. We analyze their influence on the elastic properties and rupture behavior and we discuss possible experimental consequences. Finally, we numerically investigate specific sets of parameters corresponding to specific phages such as phi29 and cowpea chlorotic mottle virus (CCMV). From the experimentally measured spring constants we make predictions about specific material parameters (such as bending rigidity and Young's modulus) for both empty and filled capsids.

Geometrical ordering of DNA in bacteria.

The bacterium Caulobacter crescentus shows a remarkable spatial ordering of its chromosome that leads to a strong linear correlation between the position of genes on the chromosomal map and their spatial position in the cellular volume. In a recent study we have shown that a robust and universal geometrical ordering mechanism can explain this correlation. We demonstrated that self-avoidance of DNA, specific positioning of one or few DNA loci (such as origin or terminus) together with the action of DNA compaction proteins (that organize the chromosome into topological domains) are sufficient to get a linear arrangement of the chromosome along the cell axis. This configuration, however, only represents the population average. Individual cells can have DNA arrangements that deviate significantly from the mean configuration and that break left-right symmetry. Symmetry breaking is stronger for longer chromosomes.

A geometrical model for DNA organization in bacteria.

Recent experimental studies have revealed that bacteria, such as C. crescentus, show a remarkable spatial ordering of their chromosome. A strong linear correlation has been found between the position of genes on the chromosomal map and their spatial position in the cellular volume. We show that this correlation can be explained by a purely geometrical model. Namely, self-avoidance of DNA, specific positioning of one or few DNA loci (such as origin or terminus) together with the action of DNA compaction proteins (that organize the chromosome into topological domains) are sufficient to get a linear arrangement of the chromosome along the cell axis. We develop a Monte-Carlo method that allows us to test our model numerically and to analyze the dependence of the spatial ordering on various physiologically relevant parameters. We show that the proposed geometrical ordering mechanism is robust and universal (i.e. does not depend on specific bacterial details). The geometrical mechanism should work in all bacteria that have compacted chromosomes with spatially fixed regions. We use our model to make specific and experimentally testable predictions about the spatial arrangement of the chromosome in mutants of C. crescentus and the growth-stage dependent ordering in E. coli.

Mechanical limits of viral capsids.

We studied the elastic properties and mechanical stability of viral capsids under external force-loading with computer simulations. Our approach allows the implementation of specific geometries corresponding to specific phages, such as phi29 and cowpea chlorotic mottle virus. We demonstrate how, in a combined numerical and experimental approach, the elastic parameters can be determined with high precision. The experimentally observed bimodality of elastic spring constants is shown to be of geometrical origin, namely the presence of pentavalent units in the viral shell. We define a criterion for capsid breakage that explains well the experimentally observed rupture. From our numerics we find a crossover from gamma(2/3) to gamma(1/2) for the dependence of the rupture force on the Föppl-von Kármán number, gamma. For filled capsids, high internal pressures lead to a stronger destabilization for viruses with buckled ground states versus viruses with unbuckled ground states. Finally, we show how our numerically calculated energy maps can be used to extract information about the strength of protein-protein interactions from rupture experiments.