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1.
Braz. j. med. biol. res ; 38(6): 813-823, June 2005. ilus, tab
Article in English | LILACS | ID: lil-402669

ABSTRACT

Enveloped viruses always gain entry into the cytoplasm by fusion of their lipid envelope with a cell membrane. Some enveloped viruses fuse directly with the host cell plasma membrane after virus binding to the cell receptor. Other enveloped viruses enter the cells by the endocytic pathway, and fusion depends on the acidification of the endosomal compartment. In both cases, virus-induced membrane fusion is triggered by conformational changes in viral envelope glycoproteins. Two different classes of viral fusion proteins have been described on the basis of their molecular architecture. Several structural data permitted the elucidation of the mechanisms of membrane fusion mediated by class I and class II fusion proteins. In this article, we review a number of results obtained by our laboratory and by others that suggest that the mechanisms involved in rhabdovirus fusion are different from those used by the two well-studied classes of viral glycoproteins. We focus our discussion on the electrostatic nature of virus binding and interaction with membranes, especially through phosphatidylserine, and on the reversibility of the conformational changes of the rhabdovirus glycoprotein involved in fusion. Taken together, these data suggest the existence of a third class of fusion proteins and support the idea that new insights should emerge from studies of membrane fusion mediated by the G protein of rhabdoviruses. In particular, the elucidation of the three-dimensional structure of the G protein or even of the fusion peptide at different pH's might provide valuable information for understanding the fusion mechanism of this new class of fusion proteins.


Subject(s)
Animals , Humans , Glycoproteins/physiology , Membrane Fusion/physiology , Rhabdoviridae/physiology , Viral Fusion Proteins/physiology , GTP-Binding Proteins/physiology , Histidine/physiology , Membrane Glycoproteins/physiology , Phosphatidylserines/physiology
2.
Braz. j. med. biol. res ; 38(6): 807-812, June 2005. ilus
Article in English | LILACS | ID: lil-402670

ABSTRACT

Apoptosis is the most common phenotype observed when cells die through programmed cell death. The morphologic and biochemical changes that characterize apoptotic cells depend on the activation of a diverse set of genes. Apoptosis is essential for multicellular organisms since their development and homeostasis are dependent on extensive cell renewal. In fact, there is strong evidence for the correlation between the emergence of multicellular organisms and apoptosis during evolution. On the other hand, no obvious advantages can be envisaged for unicellular organisms to carry the complex machinery required for programmed cell death. However, accumulating evidence shows that free-living and parasitic protozoa as well as yeasts display apoptotic markers. This phenomenon has been related to altruistic behavior, when a subpopulation of protozoa or yeasts dies by apoptosis, with clear benefits for the entire population. Recently, phosphatidylserine (PS) exposure and its recognition by a specific receptor (PSR) were implicated in the infectivity of amastigote forms of Leishmania, an obligatory vertebrate intramacrophagic parasite, showing for the first time that unicellular organisms use apoptotic features for the establishment and/or maintenance of infection. Here we focus on PS exposure in the outer leaflet of the plasma membrane - an early hallmark of apoptosis - and how it modulates the inflammatory activity of phagocytic cells. We also discuss the possible mechanisms by which PS exposure can define Leishmania survival inside host cells and the evolutionary implications of apoptosis at the unicellular level.


Subject(s)
Animals , Apoptosis/physiology , Leishmania/physiology , Phosphatidylserines/physiology , Apoptosis/immunology , Arginase/metabolism , Host-Parasite Interactions/immunology , Host-Parasite Interactions/physiology , Immune System/physiology , Leishmania/immunology , Macrophages/physiology , Phosphatidylserines/immunology
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