Human immunodeficiency virus type 1 Nef-induced CD4 cell surface downregulation is inhibited by ikarugamycin

One well-characterized in vitro function of Nef is its ability to remove CD4, the human immunodeficiency virus (HIV) receptor, from the cell surface. Nef accomplishes this by accelerating the internalization and degradation of CD4. Current models propose that Nef promotes CD4 internalization via an increased association of CD4 with clathrin-coated pits (CCP). Here, we investigated the effect of a naturally occurring antiprotozoan antibiotic, ikarugamycin (IKA), on CD4 cell surface expression in human monocytic cells stably expressing HIV type 1 SF2 Nef. IKA was able to efficiently restore CD4 cell surface expression in Nef-expressing cells without affecting either CD4 synthesis or Nef expression. In addition, we demonstrate that IKA is also capable of efficiently blocking CD4 down-modulation in response to phorbol myristate acetate. Our data suggest that IKA may be an efficient and useful inhibitor of CCP-dependent endocytosis.

Lentivirus Nef specifically activates Pak2

Nef proteins from human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) have been found to associate with an active cellular serine/threonine kinase designated Nef-associated kinase (Nak). The exact identity of Nak remains controversial, with two recent studies indicating that Nak may be either Pak1 or Pak2. In this study, we investigated the hypothesis that such discrepancies arise from the use of different Nef alleles or different cell types by individual investigators. We first confirm that Pak2 but not Pak1 is cleaved by caspase 3 in vitro and then demonstrate that Nak is caspase 3 sensitive, regardless of Nef allele or cell type used. We tested nef alleles from three lentiviruses (HIV-1 SF2, HIV-1 NL4-3, and SIVmac239) and used multiple cell lines of myeloid, lymphoid, and nonhematopoietic origin to evaluate the identity of Nak. We demonstrate that ectopically expressed Pak2 can substitute for Nak, while ectopically expressed Pak1 cannot. We then show that Nef specifically mediates the robust activation of ectopically expressed Pak2, directly demonstrating that Nef regulates Pak2 activity and does not merely associate with activated Pak2. We report that most of the active Pak2 is found bound to Nef, although a fraction is not. In contrast, only a small amount of Nef is found associated with Pak2. We conclude that Nak is Pak2 and that Nef specifically mediates Pak2 activation in a low-abundance complex. These results will facilitate both the elucidation of the role of Nef in pathogenesis and the development of specific inhibitors of this highly conserved function of Nef.

Attenuation of recombinant vesicular stomatitis viruses encoding mutant glycoproteins demonstrate a critical role for maintaining a high pH threshold for membrane fusion in viral fitness

A plasmid-based recovery system was used to generate four unique vesicular stomatitis virus (VSV) mutants that encode glycoproteins (G proteins) with single or double amino acid substitutions in two conserved acidic residues adjacent to the putative G protein fusion domain. Previously we demonstrated that three of the mutant G proteins (D137-L, E139-L, and DE-SS) have slightly reduced pH thresholds for membrane fusion activity. In this report we show that even though the viruses encoding D137-L, E139-L, and DE-SS were recovered with high efficiency, these mutants were attenuated for growth in cell culture. Plaque formation was significantly delayed with these mutants and the plaques were smaller and more diffuse than those produced by wild type VSV. In addition, cells infected with these mutants produced approximately 5- to 10-fold less infectious virus than cells infected with a similarly recovered VSV encoding the wild-type G protein. Using R18-labeled virus we found that the mutant G proteins had approximately 50% of the fusion activity of wild-type G at pH 6.3 and only 75% activity at pH 5.8. We also show that the mutant viruses were more sensitive to chloroquine inhibition of infection than either wild-type VSV or the mutant E139-T, which has a fusion phenotype similar to wild-type G protein. Reduced fusion activity and attenuation of infectivity was not due to differences in the amount of G protein incorporated into virions, nor to differences in the amount of virus binding to cells at physiological pH. Although infectivity was assayed at neutral pH, we observed an increase in virus binding with both mutant and wild-type virions as the pH was lowered, and the increase in binding occurred near the pH threshold for membrane fusion activity. From these data we propose a model in which VSV entry involves an increase in virus binding to the inner leaflet of the endosomal membrane during endosome acidification. Concomitant with this higher affinity binding, G protein becomes primed to initiate fusion of the viral envelope with the endosomal membrane. Viruses with mutations that delay the onset of increased binding and fusion lag behind wild-type VSV in their ability to initiate a productive infection, potentially because the location within the cytoplasm where these viruses ultimately fuse is not optimal for either virus uncoating or replication of the viral genome.

Mutations at two conserved acidic amino acids in the glycoprotein of vesicular stomatitis virus affect pH-dependent conformational changes and reduce the pH threshold for membrane fusion

Recently, we had shown that amino acid substitutions at residues 124, 127, and 133 either abolished or drastically altered the fusion activity of the glycoprotein (G protein) of vesicular stomatitis virus (VSV), indicating that this region is important for membrane fusion and may constitute an internal fusion domain. In this report we show that amino acid substitutions at two conserved acidic residues located at the C-terminal end of the putative fusion domain also affect the fusion activity of G protein. Two substitutions, D127-L and E139-L, slightly reduced the pH threshold at which G protein mediates membrane fusion. However, both D137-L and E139-L had fusion activities equivalent to that of wild-type G protein after exposure to a pH of 5.7 or below. To determine if the fusion activity of G protein required an acidic residue in this region both D137 and E139 were replaced with serine. This double substitution also reduced the pH threshold for fusion activity, but the effect appeared to be less severe than that for either of the single substitutions. The mutated G protein cDNAs were also introduced into a VSV minigenome to examine the effect of the substitutions on virus assembly and infectivity. All three mutant G proteins assembled into particles and these virions were infectious despite the altered fusion activities of the mutant G proteins. Although particles containing the mutant proteins were infectious they appeared to be attenuated, suggesting that these two acidic residues, which are conserved in several distantly related rhabdoviruses, play an important role in maintaining virus fitness.

Vesicular stomatitis virus glycoprotein mutations that affect membrane fusion activity and abolish virus infectivity

We have introduced amino acid substitutions into two regions of the extracellular domain of the vesicular stomatitis virus (VSV) glycoprotein (G protein) and examined the effect of these mutations on protein transport, low-pH-induced stability of G protein oligomers, and membrane fusion activity. We suggested previously that the region between amino acids 118 and 139 may be important for the membrane fusion activity of G protein, on the basis of the characterization of a fusion-defective G protein mutant (M. A. Whitt, P. Zagouras, B. Crise, and J. K. Rose, J. Virol. 64:4907-4913, 1990). It has also been postulated by others that this region as well as the region between amino acids 181 and 212 may constitute putative internal fusion domains of VSV G protein. In this report, we show that three different amino acids substitutions between residues 118 and 139 (G-124-->E, P-127-->D, and A-133-->K) either altered or abolished low-pH-dependent membrane fusion activity. In contrast, substitutions between residues 192 and 212 resulted either in G proteins that had wild-type fusion activity or in mutant proteins in which the mutation prevented transport of G protein to the cell surface. Two of the substitutions between residues 118 and 139 (G-124-->E and P-127-->D) resulted in G proteins that were fusion defective at pH 5.7, although syncytia were observed after cells were treated with fusion buffer at pH 5.5, albeit at levels significantly less than that induced by wild-type G protein. Interestingly, when either G-124-->E or P-127-->D was incorporated into tsO45 virions, the resulting particles were not infectious, presumably because the viral envelope was not able to fuse with the proper intracellular membrane. These results support the hypothesis that the region between amino acids 118 and 139 is important for the membrane fusion activity of VSV G protein and may constitute an internal fusion domain.