Molecular chaperones and the biogenesis of mitochondria and peroxisomes

Summary— A review of the proteinaceous machinery involved in protein sorting pathways and protein folding and assembly in mitochondria and peroxisomes is presented. After considering the various sorting pathways and targeting signals of mitochondrial and peroxisomal proteins, we make a comparative dissection of the protein factors involved in: i) the stabilization of cytosolic precursor proteins in a translocation competent conformation; ii) the membrane import apparatus of mitochondria and peroxisomes; iii) the processing of mitochondrial precursor proteins, and the eventual processing of certain peroxisomal precursor, in the interior of the organelles; and iv) the requirement of molecular chaperones for appropriate folding and assembly of imported proteins in the matrix of both organelles. Those aspects of mitochondrial biogenesis that have developed rapidly during the last few years, such as the requirement of molecular chaperones, are stressed in order to stimulate further parallel investigations aimed to understand the origin, biochemistry, molecular biology and pathology of peroxisomes. In this regard, a brief review of findings from our group and others is presented in which the role of the F₁-ATPase α-subunit is pointed out as a molecular chaperone of mitochondria and chloroplasts. In addition, data are presented that could question our previous indication that the immunoreactive protein found in the rat liver peroxisomes is due to the presence of the F₁-ATPase α-subunit.     

Temperature-dependent sex determination in reptiles: Proximate mechanisms,ultimate outcomes,and practical applications

In many egg-laying reptiles, the incubation temperature of the egg determines the sex of the offspring, a process known as temperature-dependent sex determination (TSD). In TSD sex determination is an “all or none” process and intersexes are rarely formed. How is the external signal of temperature transduced into a genetic signal that determines gonadal sex and channels sexual development? Studies with the red-eared slider turtle have focused on the physiological, biochemical, and molecular cascades initiated by the temperature signal. Both male and female development are active processes—rather than the crganized/default system characteristic of vertebrates with genotypic sex determination—that require simultaneous activation and suppression of testis- and ovary-determining cascades for normal sex determination. It appears that temperature accomplishes this end by acting on genes encoaing for steroidogenic enzymes and steroid hormone receptors and modifying the endocrine microenvironment in the embryo. The temperature experienced in development also has long-term functional outcomes in addition to sex determination. Research with the leopard gecko indicates that incubation temperature as well as steroid hormones serve as organizers in shaping the adult phenotype, with temperature modulating sex hormone action in sexual differentiation. Finally, practical applications of this research have emerged for the conservation and restoration of endangered egg-laying reptiles as well as the embryonic development of reptiles as biomarkers to monitor the estrogenic effects of common environmental contaminants. © 1994 Wiley-Liss, Inc.     

Sexual isolation in Drosophila. II. Intraspecific variation in mate recognition systems

A simple behavioral model is used to investigate whether differences in the specific-mate-recognition system (SMRS), occur within species of the Drosophila genus. This model takes into account, and overcomes, the distorting effect of vigor differences on experimental results. Analysis shows significant deviations from the expected values under the assumption of identical SMRSs in around one fifth of the multiple-choice experiments performed with natural strains of twelve different Drosophila species. Different selection procedures raise the number of significant assortative mating results between strains of D. melanogaster and D. pseudoobscura from 3.0% to 32.8%. Finally, sub- or semispecific taxa show variations in their SMRS even more frequently (74.5%). Differences in male vigor and female receptivity are also found. These results show that a classification of Drosophila species based on SMRS stability, as proposed by the “Recognition concept of species”, is virtually impossible.     

DEFORMATIVE DISEASE IN IRIDAEA LAMINARIOIDES (RHODOPHYTA): GALL DEVELOMENT ASSOCIATED WITH AN ENDOPHYTIC CYANOBACTERIUM1

This study is the first report of an algal disease, developed in close association with an endophytic organism, documented for the southeastern Pacific. We describe a disease affecting wild populations of the red alga Iridaea laminarioides Bory in central Chile, characterized by gall development on the surface of sporophytic, cystocarpic, and immature thalli. These abnormal growths result in severe morphological alterations of the affected thalli. Diseased fronds display an aggregated spatial distribution and occur throughout the year, with a maximum in summer followed by a decline in winter. The presence of galls was not associated with broken or torn fronds. Although causality has not been unequivocally demonstrated, our field and laboratory observations indicate a strong association of the galls with infections by an endophytic cyanobacterium, probably belonging to the genus Pleurocapsa.     

Immunocytochemical localization of DNA in synaptonemal complexes of rat and mouse spermatocytes,and of chick oocytes

The distribution of DNA in synaptonemal complexes of rat and mouse spermatocytes, and of chick oocytes was investigated by immunogold electron microscopy. Except for a few specific sites, DNA was not immunolocalized in the space between lateral elements of the complex. Some labeled fibrils connecting the lateral elements with the central element were observed associated with recombination nodules or near them. However, other labeled fibrils in the space between lateral elements did not appear to present any relationship to recombination nodules. The immunocytochemical approaches used here confirmed the presence of significant amounts of DNA in the lateral elements as previously indicated by preferential DNA staining methods. Furthermore, our findings support the view that recombination nodules are the site of chiasma formation.     

RNA from LPS-stirnulated macrophages induces the release of tumour necrosis factor-alpha and interleukin-1 by resident macrophages

The effect of exogenous RNA on many cellular functions has been studied in a variety of eukaryotic cells but there are few reports on macrophages. In the present study, it is demonstrated that cytoplasmatic RNA extracted from rat macrophages stimulated with Escherichia coli lipopolysaccharide (LPS), referred to as L-RNA, induced the release of TNF-alpha and IL-1 from monolayers of peritoneal resident macrophages. The activity of L-RNA was not altered by polymyxin B but was abolished by ribonuclease (RNase) pretreatment, indicating the absence of LPS contamination and that the integrity of the polynucleotide chain is essential for this activity. Both the poly A(-) and poly A(+) fractions obtained from L-RNA applied to oligo(dT)-cellulose chromatography induced TNF-alpha and IL-1 release. The L-RNA-induced cytokine release was inhibited by dexamethasone and seemed to be dependent on protein synthesis since this effect was abolished by cycloheximide or actinomycin-D. The LPS-stimulated macrophages, when pre-incubated with [5-(3)H]-uridine, secreted a trichloroacetic acid (TCA) precipitable material which was sensitive to RNase and KOH hydrolysis, suggesting that the material is RNA. This substance was also released from macrophage monolayers stimulated with IL-1beta but not with TNF-alpha, IL-6 or IL-8. The substance secreted ((3)H-RNA) sediments in the 4-5S region of a 5-20% sucrose gradient. These results show that L-RNA induces cytokine secretion by macrophage monolayers and support the idea that, during inflammation, stimulated macrophages could release RNA which may further induce the release of cytokines by the resident cell population.     

Comparison of conformational characteristics in structurally similar protein pairs.

Although it is known that three-dimensional structure is well conserved during the evolutionary development of proteins, there have been few studies that consider other parameters apart from divergence of the main-chain coordinates. In this study, we align the structures of 90 pairs of homologous proteins having sequence identities ranging from 5 to 100%. Their structures are compared as a function of sequence identity, including not only consideration of C alpha coordinates but also accessibility, Ooi numbers, secondary structure, and side-chain angles. We discuss how these properties change as the sequences become less similar. This will be of practical use in homology modeling, especially for modeling very distantly related or analogous proteins. We also consider how the average size and number of insertions and deletions vary as sequences diverge. This study presents further quantitative evidence that structure is remarkably well conserved in detail, as well as at the topological level, even when the sequences do not show similarity that is significant statistically.     

MACROALGAE SURVIVE DIGESTION BY FISHES1

Small individuals (<15 cm long) of the clingfish Sicyases sanguineus consume as many as 18 seaweed species in Central Chile. Enteromorpha sp. was the only species able to survive digestion by fish recently collected from the field. Enteromorpha compressa and Gelidium chilense also survived by tissue regeneration when offered as food in the laboratory, but only E. compressa showed stimulation of swarmer production. In general, our results on a vertebrate species reproduce previous findings on diverse invertebrate species. Since fish are highly motile, their capacity to disperse algal species by defecation might be of great ecological importance.     

DIGESTION SURVIVAL OF ALGAE: SOME ECOLOGICAL COMPARISIONS BETWEEN FREE SPORES AND PROPAGULES IN FECAL PELLETS1

Benthic algae survive passage through the digestive tract of different types of grazers, either regenerating new tissues or releasing protoplasts and swarmers which act as accessory means of reproduction to generate new individuals. The ecological importance of this phenomenon is evaluated here by studying spores and fecal pellets. Alga fragments egested alive inside fecal pellets have some ecological advantages over free propagules. In intertidal habitats, the sticky nature of the pellets permits attachment to the substratum while protecting the contained algae from desiccation. In subtidal habitats, the pellets sink 8–22 times faster than the fastest sinking algal propagule. Quantification of swarmers and protoplasts released from algal fragments in the fecal pellets indicates densities of about 217 germlings·cm^?2 and values of 300–700 propagules per pellet. Extrapolation of these results to field conditions based on number of grazers and pellets suggest production figures varying from 0.5–3.5 × 10⁵to 1.3–1.5 × 10⁶ propagules·m^?2·12 h^?1. These values are within known density ranges of settling spores in the field. The ecological significance of the phenomenon may be especially important at seasons when the density of grazers increases and the normal reproductive activity of the algae being consumed decreases.