The prefusogenic intermediate of HIV-1 gp41 contains exposed C-peptide regions

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein is composed of a complex between the surface subunit gp120, which binds to cellular receptors, and the transmembrane subunit gp41. Upon activation of the envelope glycoprotein by cellular receptors, gp41 undergoes conformational changes that mediate fusion of the viral and cellular membranes. Prior to formation of a fusogenic "trimer-of-hairpins" structure, gp41 transiently adopts a prefusogenic conformation whose structural features are poorly understood. An important approach toward understanding structural conformations of gp41 during HIV-1 entry has been to analyze the structural targets of gp41 inhibitors. We have constructed epitope-tagged versions of 5-Helix, a designed protein that binds to the C-peptide region of gp41 and inhibits HIV-1 membrane fusion. Using these 5-Helix variants, we examined which conformation of gp41 is the target of 5-Helix. We find that although 5-Helix binds poorly to native gp41, it binds strongly to gp41 activated by interaction of the envelope protein with either soluble CD4 or membrane-bound cellular receptors. This preferential interaction with activated gp41 results in the accumulation of 5-Helix on the surface of activated cells. These results strongly suggest that the gp41 prefusogenic intermediate is the target of 5-Helix and that this intermediate has a remarkably "open" structure, with exposed C-peptide regions. These results provide important structural information about this intermediate that should facilitate the development of HIV-1 entry inhibitors and may lead to new vaccine strategies.     

Solution structure of the DFF-C domain of DFF45/ICAD. A structural basis for the regulation of apoptotic DNA fragmentation

DFF45/ICAD has dual functions in the final stage of apoptosis, by acting as both a folding chaperone and a DNase inhibitor of DFF40/CAD. Here, we present the solution structure of the C-terminal domain of DFF45, which is essential for its chaperone-like activity. The structure of this domain (DFF-C) consists of four alpha helices, which are folded in a novel helix-packing arrangement. The 3D structure reveals a large cluster of negatively charged residues on the molecular surface of DFF-C. This observation suggests that charge complementation plays an important role in the interaction of DFF-C with the positively charged catalytic domain of DFF40, and thus for the chaperone activity of DFF45. The structure of DFF-C also provides a rationale for the loss of the chaperone activity in DFF35, a short isoform of DFF45. Indeed, in DFF35, the amino acid sequence is truncated in the middle of the second alpha helix constituting the structure of DFF-C, and thus both the hydrophobic core and the cluster of negative charges are disrupted.     

The effects of aggregation-inducing motifs on amyloid formation of model proteins related to neurodegenerative diseases

To examine the effects of aggregation-inducing motifs related to neurodegenerative diseases on amyloid formation of host protein, we prepared several chimera myoglobins, in which various aggregation-inducing motifs were inserted. The focused aggregation-inducing motifs included five (R5) or two (R2) oligopeptide repeats in yeast Sup35p, five octapeptide repeats (OPR) in the human prion protein, a nonamyloid beta component (NAC) in alpha-synuclein, and tandem repeats of 50 glutamines (Q50). Circular dichroism and infrared spectroscopies suggested that the OPR, R5, and Q50 motifs formed an antiparallel beta sheet as well as a random coil, whereas the R2 and NAC motifs mainly formed random coils. The OPR, R5, and Q50 mutants, but not the R2 and NAC mutants, readily formed the SDS-resistant aggregates under physiological condition, and electron microscopy revealed that the aggregates contained amyloid fibrils. The destabilization and increase in gyration radius of the OPR, R5, and Q50 mutants correlated with the tendency to form amyloid fibrils. A control mutant bearing a nonamyloidgenic sequence was also moderately destabilized but did not form amyloid fibrils. Therefore, we concluded that the OPR, R5, and Q50 motifs, even in a quite stable protein such as myoglobin, led the host protein to formation of amyloid fibrils under physiological condition.     

Functional expression of two Arabidopsis aldehyde oxidases in the yeast Pichia pastoris

To investigate the biochemical and enzymatic properties of two aldehyde oxidase (AO) isoforms of Arabidopsis thaliana, we expressed AAO1 and AAO2 cDNAs in a heterologous yeast (Pichia pastoris) system and successfully obtained the proteins in active forms. The expressed AAO1 and AAO2 proteins gave activity bands with the same mobilities on native gel electrophoresis and exhibited the same substrate preferences on zymograms with 8 aldehydes as those of AOalpha and AOgamma in Arabidopsis seedlings, respectively. Furthermore, anti-AAO1 and anti-AAO2 antibodies, which specifically recognize the seedling AOalpha and AOgamma, respectively, reacted with the AAO1 and AAO2 proteins produced in P. pastoris, respectively. These results indicate that these AO proteins are accurately produced in the yeast system, as in Arabidopsis seedlings. Using AO preparations from P. pastoris, the enzymatic properties of Arabidopsis AOalpha and AOgamma were investigated. AOalpha showed a relatively wide substrate specificity for 7 aldehydes tested, with high affinity to benzaldehyde and indole-3-aldehyde, while AOgamma could most efficiently oxidize naphthaldehyde. AOalpha was strongly inhibited by iodoacetate and KCN, while AOgamma was inhibited not only by iodoacetate and KCN but also by 2-mercaptethanol, dithiothreitol, menadion, and estradiol. AOalpha and AOgamma showed the highest activity at around 65 and 50 degrees C, respectively, and exhibited pH dependence around pH 8.0. These results indicate that the two AO isoforms in Arabidopsis seedlings have different enzymatic properties and may have different physiological roles in vivo.