Protein H1: a role for chromatin structure in the regulation of bacterial gene expression and virulence?

There has been a recent revival of interest in one of the most abundant Escherichia coli proteins, H1 (also called H-NS). This protein was first identified many years ago as a major component of the bacterial nucleoid, and has been characterized biochemically by several groups. However, no clear function for the protein emerged from these studies. Our thinking has been transformed by recent findings which complement the biochemistry with genetic data. Several mutations, selected over many years by virtue of their diverse effects on gene expression, have turned out to be allelic and to fall within the structural gene for H1. Bringing together the genetics and the biochemistry has demonstrated that the whole is worth more than the sum of the parts! These findings have far-reaching implications for the mechanisms by which gene expression is regulated and also, perhaps, for the control of bacterial virulence.     

A search for protein cores in chromosomes: Is the scaffold an artifact?

A protein chromosome scaffold structure has been proposed that acts as a structural framework for attachment of chromosomal DNA. There are several troubling aspects of this concept: (1) such structures have not been seen in many previous thin-section and whole-mount electron microscopy studies of metaphase chromosomes, while they are readily seen in leptotene and zygotene chromosomes; (2) such a structure poses problems for sister chromatid exchanges; and (3) the published photographs show a marked variation in the amount of scaffold in different whole-mount preparations. An alternative explanation is that the scaffold in whole-mount preparations represents incomplete dispersion of the high concentration of chromatin in the center of chromosomes, and when the histones are removed and the DNA dispersed, the remaining nonhistone proteins (NHPs) aggregate to form a chromosome-shaped structure. Two studies were done to determine if the scaffold is real or an artifact: (1) Chinese hamster mitotic cells and isolated chromosomes were examined using two protein stains -EDTA-regressive staining and phosphotungstic acid (PTA) stain. The EDTA-regressive stain showed ribonucleoprotein particles at the periphery of the chromosomes but nothing at the center of the chromosomes. The PTA stain showed the kinetochore plates but no central structures; and (2) isolated chromosomes were partially dispersed to decrease the high concentration of chromatin in the center of the chromosome, then treated with 4 M ammonium acetate or 2 M NaCl to dehistonize them and disperse the DNA. Under these circumstances, no chromosome scaffold was seen. We conclude that the scaffold structure is an artifact resulting from incomplete dispersion of central chromatin and aggregation of NHPs in dehistonized chromosomes.     

Ageing and the regulation of protein synthesis: a balancing act?

Ageing in diverse species ranging from yeast to humans is associated with extensive changes in both general and specific protein synthesis. Accumulating evidence now indicates that these alterations are not simply a corollary of the ageing process but, rather, they have a causative role in senescent decline. Indeed, interfering with mRNA translation significantly influences longevity. Interestingly, the mechanisms that control mRNA translation interface with intricate, conserved signalling pathways and specific conditions that regulate ageing, such as the insulin-insulin growth factor 1 signalling pathway and caloric restriction. This emerging relationship reveals that protein synthesis is a novel determinant of ageing in diverse organisms such as yeast, worms, flies and mice and can thus be considered as a universal component of the ageing process.     

Cytokinesis: an emerging unified theory for eukaryotes?

In animal and fungal cells, cytokinesis involves an actomyosin ring that forms and contracts at the division plane. Important new details have emerged concerning the composition, assembly, and dynamics of these contractile rings. In addition, recent advances suggest that targeted membrane addition is a central feature of cytokinesis in animal cells - as it is in fungi and plants - and the coordination of actomyosin ring function with targeted exocytosis at the cleavage plane is being explored. Important new information has also emerged about the spatial and temporal regulation of cytokinesis, especially in relation to the function of the spindle midzone in animal cells and the control of cytokinesis by GTPase systems.     

Antimicrobial peptides: pore formers or metabolic inhibitors in bacteria?

Antimicrobial peptides are an abundant and diverse group of molecules that are produced by many tissues and cell types in a variety of invertebrate, plant and animal species. Their amino acid composition, amphipathicity, cationic charge and size allow them to attach to and insert into membrane bilayers to form pores by 'barrel-stave', 'carpet' or 'toroidal-pore' mechanisms. Although these models are helpful for defining mechanisms of antimicrobial peptide activity, their relevance to how peptides damage and kill microorganisms still need to be clarified. Recently, there has been speculation that transmembrane pore formation is not the only mechanism of microbial killing. In fact several observations suggest that translocated peptides can alter cytoplasmic membrane septum formation, inhibit cell-wall synthesis, inhibit nucleic-acid synthesis, inhibit protein synthesis or inhibit enzymatic activity. In this review the different models of antimicrobial-peptide-induced pore formation and cell killing are presented.     

Chaperoning prions: the cellular machinery for propagating an infectious protein?

Newly made polypeptide chains require the help of molecular chaperones not only to rapidly reach their final three-dimensional forms, but also to unfold and then correctly refold them back to their biologically active form should they misfold. Most prions are an unusual type of protein that can exist in one of two stable conformations, one of which leads to formation of an infectious alternatively folded form. Studies in Baker's yeast (Saccharomyces cerevisiae) have revealed that prions can exploit the molecular chaperone machinery in the cell in order to ensure stable propagation of the infectious, aggregation-prone form. The disaggregation of yeast prion aggregates by molecular chaperones generates forms of the prion protein that can seed the protein polymerisation that underlies the prion propagation cycle. In this article, we review what we have learnt about the role of molecular chaperones in yeast prion propagation, describe a model that can explain the role of various classes of molecular chaperones and their co-chaperones, and speculate on the possible involvement of chaperones in the propagation of mammalian prions.     

Pigment-protein architecture in the light-harvesting antenna complexes of purple bacteria: does the crystal structure reflect the native pigment-protein arrangement?

Structural analysis of crystallized peripheral (LH2) and core antenna complexes (LH1) of purple bacteria has revealed circular aggregates of high rotational symmetry (C₈, C₉ and C₁₆, respectively). Quantum-chemical calculations indicate that in particular the waterwheel-like arrangements of pigments should show characteristic structure-sensitive spectroscopic behavior in the near infrared absorption region. Laser-spectroscopic data obtained with non-crystallized, isolated LH2 of Rhodospirillum molischianum are in line with a highly symmetric (C₈) circular aggregate, but deviations have been found for LH2 of Rhodobacter sphaeroides and Rhodopseudomonas acidophila. For both the latter, C-shaped incomplete circular aggregates (as seen only recently in electron micrographs of crystallized LH1–reaction center complexes) may be a suitable preliminary model.