The chloroplast is the site of photosynthesis in higher plants but also functions as the center of synthesis for primary and specialized metabolites including amino acids, fatty acids, starch, and diverse isoprenoids. Mutants that disrupt aspects of chloroplast function represent valuable tools for defining structural and biochemical regulation of the chloroplast and its interplay with whole-plant structure and function. The lutescent1 (l1) and l2 mutants of tomato (Solanum lycopersicum) possess a range of chlorophyll-deficient phenotypes including reduced rates of chlorophyll synthesis during deetiolation and enhanced rates of chlorophyll loss in leaves and fruits as they age, particularly in response to high-light stress and darkness. In addition, the onset of fruit ripening is delayed in lutescent mutants by approximately 1 week although once ripening is initiated they ripen at a normal rate and accumulation of carotenoids is not impaired. The l2 locus was mapped to the long arm of chromosome 10 and positional cloning revealed the existence of a premature stop codon in a chloroplast-targeted zinc metalloprotease of the M50 family that is homologous to the Arabidopsis (Arabidopsis thaliana) gene ETHYLENE-DEPENDENT GRAVITROPISM DEFICIENT AND YELLOW-GREEN1. Screening of tomato germplasm identified two additional l2 mutant alleles. This study suggests a role for the chloroplast in mediating the onset of fruit ripening in tomato and indicates that chromoplast development in fruit does not depend on functional chloroplasts.

The tumor suppressor p53 plays a unique role as a central hub of numerous cell proliferation and apoptotic pathways and its malfunction due to mutations is a major cause of various malignancies. Therefore, it serves as an attractive target for developing novel anti-cancer therapeutics. Due to its intrinsically unstable DNA-binding domain, p53 unfolds rapidly at physiological temperature. Certain mutants shift the equilibrium towards the unfolded state and yield high-molecular-weight, non-functional, and cytotoxic β-sheet-rich aggregates that share tinctorial and conformational similarities with amyloid deposits found in various protein-misfolding diseases. Here, we examined the effect of a novel protein-assembly modulator, the lysine (Lys)-specific molecular tweezer, CLR01, on different aggregation stages of misfolded mutant p53 in vitro and on the cytotoxicity of the resulting p53 aggregates in cell culture. We found that CLR01 induced rapid formation of β-sheet-rich, intermediate-size p53 aggregates, yet inhibited further p53 aggregation and reduced cytotoxicity of the resulting aggregates. Our data suggest that aggregation modulators, such as CLR01, could prevent formation of toxic p53 aggregates.

Background:The tumor suppressor p53 is frequently mutated in cancer. Results: The p53 homolog in Drosophila is sensitive to mutations associated with human cancer, affecting its stability and target gene specificity. Conclusion:The response to mutation reflects structural and functional similarities of human and Drosophila p53. Significance: Drosophila p53 is an attractive model system for studying p53 function and mutant rescue.

Loss of function mutations in the von Hippel-Lindau (pVHL) tumor suppressor protein are tumorigenic. In silico analysis of the structure and folding of WT pVHL identified in its core an aromatic tetrahedron, essential for stabilizing the protein. The mutations disrupt the aromatic tetrahedron, leading to misfolding of pVHL. Using biophysical methods we confirmed the in silico predictions, demonstrating that mutant pVHL proteins have lower stability than the WT, distort the core domain and as a result reduce the ability of the protein to bind its target HIF-1α. Using bacterial pVHL-EGFP based assay we screened for osmolytes capable of restoring folding of mutant pVHL. Among them, Arginine was the most effective and was verified by in vitro assays as a potent re-folder of pVHL. This resulted in functional restoration of the mutant proteins to the level of the WT.

The von Hippel-Lindau (VHL) cancer syndrome is associated with mutations in the VHL gene. The pVHL protein is involved in response to changes in oxygen availability as part of an E3-ligase that targets the Hypoxia-Inducible Factor for degradation. pVHL has a molten globule configuration with marginal thermodynamic stability. The cancer-associated mutations further destabilize it. The Drosophila homolog, dVHL, has relatively low sequence similarity to pVHL, and is also involved in regulating HIF1-α. Using in silico, in vitro and in vivo approaches we demonstrate high similarity between the structure and function of dVHL and pVHL. These proteins have a similar fold, secondary and tertiary structures, as well as thermodynamic stability. Key functional residues in dVHL are evolutionary conserved. This structural homology underlies functional similarity of both proteins, evident by their ability to bind their reciprocal partner proteins, and by the observation that transgenic pVHL can fully maintain normal dVHL-HIF1-α downstream pathways in flies. This novel transgenic Drosophila model is thus useful for studying the VHL syndrome, and for testing drug candidates to treat it.

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