The skeletons of adult echinoderms comprise large single crystals of calcite with smooth convoluted fenestrated morphologies, raising many questions about how they form. By using water etching, infrared spectroscopy, electron diffraction, and environmental scanning electron microscopy, we show that sea urchin spine regeneration proceeds via the initial deposition of amorphous calcium carbonate. Because most echinoderms produce the same type of skeletal material, they probably all use this same mechanism. Deposition of transient amorphous phases as a strategy for producing single crystals with complex morphology may have interesting implications for the development of sophisticated materials.

The crystalline state is considered to be incompatible with life. However, in living systems exposed to severe environmental assaults, the sequestration of vital macromolecules in intracellular crystalline assemblies may provide an efficient means for protection. Here we report a generic defence strategy found in Escherichia coli, involving co-crystallization of its DNA with the stress-induced protein Dps. We show that when purified Dps and DNA interact, extremely stable crystals form almost instantaneously, within which DNA is sequestered and effectively protected against varied assaults. Crystalline structures with similar lattice spacings are formed in E. coli in which Dps is slightly over expressed, as well as in starved wild-type bacteria. Hence, DNA-Dps co-crystallization is proposed to represent a binding mode that provides wide-range protection of DNA by sequestration. The rapid induction and large-scale production of Dps in response to stress, as well as the presence of Dps homologues in many distantly related bacteria, indicate that DNA protection by biocrystallization may be crucial and widespread in prokaryotes.

The organization of apatite crystals and collagen fibrils in mineralized turkey tendon has been studied by electron microscopy and electron diffraction. To minimize artifactual distortions the tissue was examined, for the first time, as isolated fibrils in an aqueous environment of vitreous ice, as well as in conventionally prepared sections. The electron micrographs show that the plate-shaped apatite crystals are arranged in parallel arrays across the collagen fibrils. This provides direct evidence for highly asymmetric assembly in collagen fibrils, and, indeed, the fibrils were observed to be elongated rather than round in cross-section. There is, furthermore, a pronounced tendency for the layers of crystals to be coherently aligned in adjacent fibrils. These observations may also be important for understanding the mechanical behavior of bone at the molecular level, as such extended, aligned aggregates of flat crystals could develop into natural fracture planes in mature bone.The structure and mechanical properities of bone derive, at the molecular level, from the organized growth of carbonated calcium phosphate (apatite) crystals within a matrix of collagen fibrils and other organic components (1). Newly formed bone has a relatively low mineral content, consisting of small crystals closely associated with the collagen fibrils, whereas in denser more mature bone many crystals are larger and not obviously related to the collagen structure. Although the early formed crystals have been shown to grow with preferred orientation in gap regions between collagen molecules (2-4), much remains to be understood about the shape, orientation, and distribution of crystals within the organic matrix, as well as the factors that initiate and control their growth. We have investigated crystal distribution in mineralized turkey tendon, a tissue often regarded as a model for some portions of bone (5-10), in which almost all the mineral crystals are associated with parallel collagen fibrils. This arrangement would therefore approximate most closely to newly formed bone.Several unusual experimental procedures have been used in our studies of turkey tendon. (i) By using ultrasonication we have been able to isolate intact single fibrils of mineralized collagen in which the very thin plate-like apatite crystals are generally much better visualized than in more conventional preparations of embedded sections. (ii) These fibrils have been studied in an aqueous environment of vitreous ice, thus avoiding the dehydration and consequent distortion of unprotected fibrils in the high vacuum of the electron microscope. (iii) We have determined the orientations and relative alignment of apatite crystals over extensive regions of the tendon by examining critical features in their electron diffraction patterns. These experiments have provided insights into the assembly structure of this much-studied tissue, which our results show to be remarkably regularly organized. We have recently obtained some evidence that similar regular structures a...

The role of zinc, an essential element for normal brain function, in the pathology of Alzheimer's disease (AD) is poorly understood. On one hand, physiological and genetic evidence from transgenic mouse models supports its pathogenic role in promoting the deposition of the amyloid beta-protein (Abeta) in senile plaques. On the other hand, levels of extracellular ("free") zinc in the brain, as inferred by the levels of zinc in cerebrospinal fluid, were found to be too low for inducing Abeta aggregation. Remarkably, the release of transient high local concentrations of zinc during rapid synaptic events was reported. The role of such free zinc pulses in promoting Abeta aggregation has never been established. Using a range of time-resolved structural and spectroscopic techniques, we found that zinc, when introduced in millisecond pulses of micromolar concentrations, immediately interacts with Abeta 1-40 and promotes its aggregation. These interactions specifically stabilize non-fibrillar pathogenic related aggregate forms and prevent the formation of Abeta fibrils (more benign species) presumably by interfering with the self-assembly process of Abeta. These in vitro results strongly suggest a significant role for zinc pulses in Abeta pathology. We further propose that by interfering with Abeta self-assembly, which leads to insoluble, non-pathological fibrillar forms, zinc stabilizes transient, harmful amyloid forms. This report argues that zinc represents a class of molecular pathogens that effectively perturb the self-assembly of benign Abeta fibrils, and stabilize harmful non-fibrillar forms.

No abstract

No abstract

Evidence for a conceptually novel DNA packaging process is presented. X-ray scattering, electron microscopy, and circular dichroism measurements indicate that in the presence of positively charged micellar aggregates and flexible anionic polymers, such as negatively charged polypeptides or single-stranded RNA species, a complex is formed in which DNA molecules are partially embedded within a micellar scaffold and partially condensed into highly packed chiral structures. Based on studies of micelle-DNA and micelle-flexible anionic polymer systems, as well as on the known effects of a high charge density upon the micellar organization, a DNA packaging model is proposed. According to this model, the DNA induces the elongation of the micelles into rodlike aggregates, forming a closely packed matrix in which the DNA molecules are immobilized. In contrast, the flexible anionic polymers stabilize clusters of spherical micelles which are proposed to effect a capping of the rodlike micelles, thus arresting their elongation and creating surfactant-free segments of the DNA that are able to converge and collapse. Thus, unlike other in vitro DNA packaging systems, in which condensation follows encounters between charge-neutralized DNA molecules, a prepackaging phase where the DNA is immobilized within a matrix is proposed in this case. Cellular and nuclear membranes have been implicated in DNA packaging processes in vivo, and negatively charged polyelectrolytes were shown to be involved in the processes. These observations, combined with the basic tenets of the DNA condensation system described here, allow for the progression to the study of more elaborate model systems and thus might lead to insights into the nature and roles of the intricate in vivo DNA-membrane complexes.

No abstract