Membrane Remodeling During Viral Infection, Parasite Inv

Child Health And Human Development

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Abstract

This project is aimed at the understanding of the physico-chemical mechanisms of membrane remodeling during physiological and pathogenic events. During apoptotic cell death, cells usually release apoptogenic proteins such as cytochrome c from the mitochondrial intermembrane space. If Bcl-2 family proteins induce such release by increasing outer mitochondrial membrane permeability, then the pro-apoptotic, but not anti-apoptotic activity of these proteins should correlate with their permeabilization of membranes to cytochrome c. This year we tested this hypothesis using pro-survival full-length Bcl-xL and pro-death Bcl-xL cleavage products (DN61Bcl-xL and DN76Bcl-xL). Unlike Bcl-xL, DN61Bcl-xL and DN76Bcl-xL caused the release of cytochrome c from mitochondria in vivo and in vitro. Recombinant DN61Bcl-xL and DN76Bcl-xL, as well as Bcl-xL, cleaved in situ by caspase 3-possessed intrinsic pore-forming activity as demonstrated by their ability to efficiently permeabilize pure lipid vesicles. Furthermore, only DN61Bcl-xL and DN76Bcl-xL, but not Bcl-xL, formed pores large enough to release cytochrome c and to destabilize planar lipid bilayer membranes through reduction of pore line tension. Because Bcl-xL and its C-terminal cleavage products bound similarly to lipid membranes and formed oligomers of the same size, neither lipid affinity nor proteinprotein interactions appear to be solely responsible for the increased membrane-perturbing activity elicited by Bcl-xL cleavage. Taken together, these data are consistent with the hypothesis that pro-apoptotic forms of Bcl-2 family proteins permeabilize the outer mitochondrial membrane through a multistep process, ultimately leading to liberation of intermembrane apoptogenic factors into the cytosol. Since the molecular mechanism by which Bcl-2 prevents apoptosis still remains elusive, we have also studied recombinant human Bcl-2 with the deletion of 22 residues at the C-terminal membrane-anchoring region (rhBcl-2Delta22). Characterization of rhBcl-2Delta22 showed that the recombinant protein is homogeneous and monodisperse in nondenaturing solutions, stable at room temperature in the presence of a metal chelator, and an alpha-helical protein with unfolding of secondary structure at a T(m) of 62.8 degrees C. Optimal membrane pore formation by rhBcl-2Delta22 required negatively charged phospholipids. The existence of a hydrophobic groove in rhBcl-2Delta22 was demonstrated by the fluorescence enhancement of the hydrophobic ANS probe with which a pro-apoptotic Bak BH3 peptide competed. The respiratory inhibitor antimycin A also bound to the hydrophobic groove of rhBcl-2Delta22 with a K(d) of 0.82 microM. The optimal binding conformation of antimycin A was predicted from molecular docking of antimycin A with the hBcl-2 model created by homology modeling. Antimycin A selectively induces apoptosis in cells overexpressing Bcl-2, suggesting that hydrophobic groove-binding compounds may act as selective apoptotic triggers in tumor cells. In the infection of cells by enveloped viruses, 4-GU-DANA (zanamivir) (as well as DANA and 4-AM-DANA) was found to inhibit the neuraminidase activity of human parainfluenza virus type 3 (HPF3). The viral neuraminidase activity is attributable to hemagglutinin-neuraminidase (HN), an envelope protein essential for viral attachment and for fusion mediated by the other envelope protein, F. While there is no evidence that HN's neuraminidase activity is essential for receptor binding and syncytium formation, we found that 4-GU-DANA prevented hemadsorption and fusion of persistently infected cells with uninfected cells. In plaque assays, 4-GU-DANA reduced the number (but not the area) of plaques if present only during the adsorption period and reduced plaque area (but not number) if added only after the 90-min adsorption period. 4-GU-DANA also reduced the area of plaques formed by a neuraminidase-deficient variant, confirming that its interference with cell-cell fusion is unrelated to inhibition of neuraminidase activity. The order-of-magnitude lower 50% inhibitory concentrations of 4-GU-DANA (and also DANA and 4-AM-DANA) for plaque area reduction and for inhibition in the fusion assay than for reducing plaque number or blocking hemadsorption indicate the particular efficacy of these sialic acid analogs in interfering with cell-cell fusion. In cell lines expressing influenza virus hemagglutinin (HA) as the only viral protein, we found that 4-GU-DANA had no effect on hemadsorption but did inhibit HA2b-red blood cell fusion, as judged by both lipid mixing and content mixing. Thus, 4-GU-DANA can interfere with both influenza virus- and HPF3-mediated fusion. The results indicate that (i) in HPF3, 4-GU-DANA and its analogs have an affinity not only for the neuraminidase active site of HN but also for sites important for receptor binding and cell fusion and (ii) sialic acid-based inhibitors of influenza virus neuraminidase can also exert a direct, negative effect on the fusogenic function of the other envelope protein, HA.