Primary sooty mangabey simian immunodeficiency virus and human immunodeficiency virus type 2 nef alleles modulate cell surface expression of various human receptors and enhance viral infectivity and replication

The nef gene of the pathogenic simian immunodeficiency virus (SIV) mac239 clone has been well characterized. Little is known, however, about the function of nef alleles derived from naturally SIVsm-infected sooty mangabeys (Cercocebus atys) and from human immunodeficiency virus type 2 (HIV-2)-infected individuals. Addressing this, we demonstrate that, similarly to the SIVmac239 nef, primary SIVsm and HIV-2 nef alleles down-modulate cell surface expression of human CD4, CD28, CD3, and class I or II major histocompatibility complex (MHC-I or MHC-II, respectively) molecules, up-regulate surface expression of the invariant chain (Ii) associated with immature MHC-II, inhibit early T-cell activation events, and enhance virion infectivity. Both also stimulate viral replication, although HIV-2 nef alleles were less active in this assay than SIVsm nef alleles. Mutational analysis showed that a dileucine-based sorting motif in the C-proximal loop of SIV or HIV-2 Nef is critical for its effects on CD4, CD28, and Ii but dispensable for down-regulation of CD3, MHC-I, and MHC-II. The C terminus of SIV and HIV-2 Nef was exclusively required for down-modulation of MHC-I, further demonstrating that analogous functions are mediated by different domains in Nef proteins derived from different groups of primate lentiviruses. Our results demonstrate that none of the eight Nef functions investigated had been newly acquired after cross-species transmission of SIVsm from naturally infected mangabeys to humans or macaques. Notably, HIV-2 and SIVsm nef alleles efficiently down-modulate CD3 and C28 surface expression and inhibit T-cell activation more efficiently than HIV-1 nef alleles. These differences in Nef function might contribute to the relatively low levels of immune activation observed in HIV-2-infected human individuals.     

Molecular systematics and paleobiogeography of the South American sigmodontine rodents [published erratum appears in Mol Biol Evol 1998 Feb;15(2):224]

The murid rodent subfamily Sigmodontinae contains 79 genera which are distributed throughout the New World. The time of arrival of the first sigmodontines in South America and the estimated divergence time(s) of the different lineages of South American sigmodontines have been controversial due to the lack of a good fossil record and the immense number of extant species. The "early-arrival hypothesis" states that the sigmodontines must have arrived in South America no later than the early Miocene, at least 20 MYA, in order to account for their vast present-day diversity, whereas the "late-arrival hypothesis" includes the sigmodontines as part of the Plio-Pleistocene Great American Interchange, which occurred approximately 3.5 MYA. The phylogenetic relationships among 33 of these genera were reconstructed using mitochondrial DNA (mtDNA) sequence data from the ND3, ND4L, arginine tRNA, and ND4 genes, which we show to be evolving at the same rate. A molecular clock was calibrated for these genes using published fossil dates, and the genetic distances were estimated from the DNA sequences in this study. The molecular clock was used to estimate the dates of the South American sigmodontine origin and the main sigmodontine radiation in order to evaluate the "early-" and "late-arrival" scenarios. We estimate the time of the sigmodontine invasion of South America as between approximately 5 and 9 MYA, supporting neither of the scenarios but suggesting two possible models in which the invading lineage was either (1) ancestral to the oryzomyines, akodonts, and phyllotines or (2) ancestral to the akodonts and phyllotines and accompanied by the oryzomyines. The sigmodontine invasion of South America provides an example of the advantage afforded to a lineage by the fortuitous invasion of a previously unexploited habitat, in this case an entire continent.      

Diffusion of molecules and macromolecules in thylakoid membranes

The survival and fitness of photosynthetic organisms is critically dependent on the flexible response of the photosynthetic machinery, harbored in thylakoid membranes, to environmental changes. A central element of this flexibility is the lateral diffusion of membrane components along the membrane plane. As demonstrated, almost all functions of photosynthetic energy conversion are dependent on lateral diffusion. The mobility of both small molecules (plastoquinone, xanthophylls) as well as large protein supercomplexes is very sensitive to changes in structural boundary conditions. Knowledge about the design principles that govern the mobility of photosynthetic membrane components is essential to understand the dynamic response of the photosynthetic machinery. This review summarizes our knowledge about the factors that control diffusion in thylakoid membranes and bridges structural membrane alterations to changes in mobility and function. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.     

Resistance of short term activated T cells to CD95-mediated apoptosis correlates with de novo protein synthesis of c-FLIPshort

In the early phase of an immune response, T cells are activated and acquire effector functions. Whereas these short term activated T cells are resistant to CD95-mediated apoptosis, activated T cells in prolonged culture are readily sensitive, leading to activation-induced cell death and termination of the immune response. The translation inhibitor, cycloheximide, partially overcomes the apoptosis resistance of short term activated primary human T cells. Using this model we show in this study that sensitization of T cells to apoptosis occurs upstream of mitochondria. Neither death-inducing signaling complex formation nor expression of Bcl-2 proteins is altered in sensitized T cells. Although the caspase-8 inhibitor c-FLIP(long) was only slightly down-regulated in sensitized T cells, c-FLIP(short) became almost undetectable. This correlated with caspase-8 activation and apoptosis. These data suggest that c-FLIP(short), rather than c-FLIP(long), confers resistance of T cells to CD95-mediated apoptosis in the context of immune responses.     

The surface layer of Mycoplasma mobile 163K and its possible relevance to cell cohesion and group motility

Mycoplasma mobile strain 163K tends to move in multicellular configurations, either as pairs or small groups of three or more cells, or as chain-like aggregations or microcolonies. Such wandering groups arise by transient association of independently moving cells. This behaviour of M. mobile was microscopically investigated and documented by sequences of microcinematographic pictures, as well as by photomicrographically recorded motility tracks. The presence of an extracellular slime layer was demonstrated in thin sections, by negative staining and by scanning electron microscopy. The possible association of this layer with the cohesive properties of the mycoplasma cells, enabling the formation of wandering groups, is discussed and a calculation of the magnitude of the cohesive force is provided.     

Male-specific modification of human CD52.

CD52 is an unusually short, bipolar glycopeptide bearing a highly charged N-linked carbohydrate moiety and a glycosylphosphatidylinositol membrane anchor. It is exclusively expressed on lymphocytes and in the male genital tract where it is shed into the seminal plasma and inserts into the sperm membrane. The sperm surface molecule has potential significance as a target for antibodies that inhibit sperm function and gamete interaction. Western blot analyses suggested cell type-specific modifications of the antigen. It was purified from seminal plasma and a detailed structural analysis performed. The majority of anchor structures in male genital tract CD52 showed 2-inositol palmitoylation, rendering molecules insensitive toward phospholipase C, and a sn-1-alkyl-2-lyso-glycerol structure in place of the diacylated anchor described by Treumann et al. (Treumann, A., Lifely, M. R., Schneider, P., and Ferguson, M. A. (1995) J. Biol. Chem. 270, 6088-6099). N-Glycans of the male genital tract product were based on bi-, tri-, and tetraantennary structures of highly charged (up to -7), terminally sialylated complex-type sugars. A substantial proportion carried varying numbers of lactosamine repeats of which nearly 30% were branched. Different from lymphocytes, 10-15% of all N-glycans of the male genital tract antigen also contained peripheral fucose. These data confirm that male genital tract CD52 is distinct from the lymphocyte form by both N-linked glycans and COOH-terminal attached lipid anchor.     

Characterization of a novel mutation in the cardiac ryanodine receptor that results in catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an arrhythmogenic disease that manifests as syncope or sudden death during high adrenergic tone in the absence of structural heart defects. It is primarily caused by mutations in the cardiac ryanodine receptor (RyR2). The mechanism by which these mutations cause arrhythmia remains controversial, with discrepant findings related to the role of the RyR2 binding protein FKBP12.6. The purpose of this study was to characterize a novel RyR2 mutation identified in a kindred with clinically diagnosed CPVT.Single-strand conformational polymorphism analysis and direct DNA sequencing were used to screen the RyR2 gene for mutations. Site-directed mutagenesis was employed to introduce the mutation into the mouse RyR2 cDNA. The impact of the mutation on the interaction between RyR2 and a 12.6 kDa FK506 binding protein (FKBP12.6) was determined by immunoprecipitation and immunoblotting and its effect on RyR2 function was characterized by single cell Ca²⁺ imaging and [³H]ryanodine binding.A novel CPVT mutation, E189D, was identified. The E189D mutation does not alter the affinity of the channel for FKBP12.6, but it increases the propensity for store-overload-induced Ca²⁺ release (SOICR). Furthermore, the E189D mutation enhances the basal channel activity of RyR2 and its sensitivity to activation by caffeine.The E189D RyR2 mutation is causative for CPVT and functionally increases the propensity for SOICR without altering the affinity for FKBP12.6. These observations strengthen the notion that enhanced SOICR, but not altered FKBP12.6 binding, is a common mechanism by which RyR2 mutations cause arrhythmias.Key words: arrhythmia, calcium, death sudden, genetics, ion channels     

Utilizing a structure-based docking approach to develop potent G protein-coupled receptor kinase (GRK) 2 and 5 inhibitors

G protein-coupled receptor (GPCR) kinases (GRKs) regulate the desensitization and internalization of GPCRs. Two of these, GRK2 and GRK5, are upregulated in heart failure and are promising targets for heart failure treatment. Although there have been several reports of potent and selective inhibitors of GRK2 there are few for GRK5. Herein, we describe a ligand docking approach utilizing the crystal structures of the GRK2–Gβγ·GSK180736A and GRK5·CCG215022 complexes to search for amide substituents predicted to confer GRK2 and/or GRK5 potency and selectivity. From this campaign, we successfully generated two new potent GRK5 inhibitors, although neither exhibited selectivity over GRK2.