Ranajit Pal

Characterization of an HIV-1 point mutant blocked in envelope glycoprotein cleavage

The envelope proteins of retroviruses are derived from a polypeptide precursor protein by cleavage adjacent to a cluster of basic amino acids. Site-specific mutagenesis was used to construct a mutant of the human immunodeficiency virus type 1 (HIV-1) in which the arginine residue at the carboxy-terminus of the gp120 was changed to a threonine residue. This single substitution was sufficient to abolish all detectable cleavage of the gp160 envelope precursor polypeptide as well as virus infectivity. The gp160 was produced in normal quantities from a biologically active clone of the mutant virus after transfection into cos-1 cells. The mutant gp160 contained N-linked oligosaccharide chains with mannose-rich cores similar to those of the gp160 produced by the wild-type clone. Immunofluorescence assays showed that gp160 was transported to the surface of transfected CD4+ HeLa cells. No envelope proteins of known size could be detected in the media of cells transfected with the mutant virus, suggesting that functional virions were not formed. Binding of the mutant gp160 to the CD4 receptor molecule was unimpaired. Despite this and the presence of gp160 on the cell surface, neither growth of mutant-transfected CD4+ HeLa cells nor cocultivation of transfected cos-1 cells with H9 cells resulted in significant syncytium formation. The data indicate that the carboxy-terminal arginine residue of HIV-1 gp120 is necessary for envelope protein cleavage and suggest cleavage is important in the virus life cycle in both functional virus release and membrane fusion.

Lipid and protein contributions to the membrane surface potential of vesicular stomatitis virus probed by a fluorescent pH indicator, 4-heptadecyl-7-hydroxycoumarin

The surface potential of membranes of vesicular stomatitis virus and liposomes was determined by shift of ionization over a wide pH range of the membrane-inserted fluorophore, 4-heptadecyl-7-hydroxycoumarin. Incorporation into sonicated vesicles of negatively charged phosphatidylserine markedly increased the surface potential of uncharged phosphatidylcholine, but no significant effect on surface potential was produced by polar but uncharged glucocerebroside incorporated in phosphatidylcholine vesicles. The membrane of vesicular stomatitis virus was found to have a moderately high surface potential. Contributing to this viral membrane surface potential were glycoprotein spikes and phospholipid headgroups as determined by lowered charge after treatment of intact virions with thermolysin to remove glycoprotein or phospholipase C to remove phospholipid headgroups. The role of viral glycoprotein was confirmed by demonstrating increased surface charge of vesicles reconstituted with both viral glycoprotein and lipids compared with vesicles reconstituted with viral lipids alone. An unexpected finding was the large contribution to surface potential of cholesterol present in viral membrane. Increasing cholesterol concentration in virions by interaction with cholesterol-complexed serum lipoproteins resulted in a marked decrease in surface potential, whereas 75% depletion of virion cholesterol by interaction with sphingomyelin-complexed serum lipoproteins resulted in a significant increase in virion membrane surface potential. Although removal of glycoprotein spikes or depletion of cholesterol causes reduction in infectivity of vesicular stomatitis virus, no direct correlation could be found between alteration in surface charge and infectivity.

Rhabdovirus Membrane and Maturation

Rhabdoviruses, particularly vesicular stomatitis (VSV), have provided an incisive and widely used system for studying the far more complicated biological membranes of eukaryotic cells. In addition to its simplicity, the VSV membrane can be produced in large amounts and is readily purified to homogeneity free of contaminating cell membranes. Lipids of the VSV membrane are derived from the plasma membrane of the host cell and form a lipid bilayer that assumes many of the characteristics of a biological unit membrane. The rhabdovirus membrane contains only two proteins, one integral and one peripheral; these viral proteins have interesting properties and have been widely used by biochemists to study the synthesis and mode of action of integral and peripheral membrane proteins of cells. Furthermore, the synthesis and assembly of the VSV membrane proteins in infected host cells have shed considerable light on the intricacies of cellular membrane-protein biogenesis.

Monoclonal antibodies to the matrix protein of vesicular stomatitis virus (New Jersey serotype) and their effects on viral transcription.

Of 33 hybridomas raised by immunization of BALB/c mice with the matrix (M) protein of the New Jersey serotype of vesicular stomatitis virus (VSV), 17 secreted monoclonal antibodies (mAb) of the IgG isotype and, unexpectedly, 16 of the IgM isotype. All these monoclonal antibodies bound strongly to VSV-New Jersey M protein by ELISA, immunoprecipitation, and immunoblotting assays, but exhibited only slight or no cross-reactivity with the M protein of VSV-Indiana. Four antigenic determinants of VSV-New Jersey M protein could be identified by competitive binding of 125I-labeled monoclonal antibodies but three of these epitopes exhibited partial overlap. Monoclonal antibodies to two epitopes reversed the inhibitory effect of M protein on in vitro transcription of VSV-New Jersey ribonucleoprotein. However, monoclonal antibodies to the other two epitopes had little effect on M-protein transcription inhibition but actually increased significantly the transcriptional inhibitory effect of M protein under certain experimental conditions. Monoclonal antibodies to all four epitopes reacted strongly with the M protein of the tsC1 mutant of VSV-New Jersey which is restricted in transcription inhibition.

Metabolic labeling of viral membrane lipids by fluorescent fatty acids : studying virus fusion with target membranes

Publisher Summary This chapter discusses metabolic labeling of viral membrane lipids by fluorescent fatty acids. Enveloped virions introduce their nucleocapsid into eukaryotic host cells by two different routes, both of which are fusion-dependent. The fusion is mediated by envelope viral proteins, although the two routes differ in their target membrane within the host cell. In most systems, the virions are introduced into cellular endosomes by receptor-mediated endocytosis. The other fusion pathway, for which paramyxoviruses are the main representatives, is characterized by receptor-mediated binding of the virions to the host cell plasma membrane and direct fusion between the virions and the host cell plasma membrane. The intermixing of contents and of membrane components serves as the basis of most fusion assays, particularly quantitative ones. Two general approaches are used to measure mixing of virion membrane components with those of its target membrane—namely, (1) mixing of probes assays and (2) dilution of probes assays.

Regulation of viral transcription by the matrix protein of vesicular stomatitis virus probed by monoclonal antibodies and temperature-sensitive mutants

The ability of the matrix (M) protein of wild-type vesicular stomatitis virus (VSV) to regulate viral transcription was studied with monoclonal antibodies and temperature-sensitive (ts) mutants in complementation group III, the M proteins of which are restricted in transcription inhibition. The marked inhibition of transcription by VSV ribonucleoprotein (RNP) cores complexed with M protein (RNP/M) was reversed by antibody to epitope 1. Antibodies to epitopes 2 and 3 not only failed to reverse the transcription-inhibitory activity of isolated M protein but actually increased M-protein inhibition of transcription in a reconstituted system. Monoclonal antibodies to epitopes 2 and 3 strongly bound to M proteins from all wild-type and ts-mutant virions, but monoclonal antibody to epitope 1 completely failed to bind to the M protein of ts023(III) even though it reacted strongly with M proteins of mutants tsG31(III) and tsG33(III). The M protein of a tsO23 revertant (R11) completely recovered its capacity to inhibit transcription and to bind monoclonal antibody to epitope 1, whereas the M proteins of three other revertants remained restricted in their capacity to inhibit transcription and to bind monoclonal antibody to epitope 1. These studies indicate that exposure of epitope 1 on the surface of M protein is essential for inhibiting transcription by VSV RNP cores.