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Mutational evidence for an internal fusion peptide in flavivirus envelope protein E.

ABSTRACT: The envelope protein E of the flavivirus tick-borne encephalitis (TBE) virus promotes cell entry by inducing fusion of the viral membrane with an intracellular membrane after uptake by endocytosis. This protein differs from other well-studied viral and cellular fusion proteins because of its distinct molecular architecture and apparent lack of involvement of coiled coils in the low-pH-induced structural transitions that lead to fusion. A highly conserved loop (the cd loop), which resides at the distal tip of each subunit and is mostly buried in the subunit interface of the native E homodimer at neutral pH, has been hypothesized to function as an internal fusion peptide at low pH, but this has not yet been shown experimentally. It was predicted by examination of the X-ray crystal structure of the TBE virus E protein (F. A. Rey et al., Nature 375:291-298, 1995) that mutations at a specific residue within this loop (Leu 107) would not cause the native structure to be disrupted. We therefore introduced amino acid substitutions at this position and, using recombinant subviral particles, investigated the effects of these changes on fusion and related properties. Replacement of Leu with hydrophilic amino acids strongly impaired (Thr) or abolished (Asp) fusion activity, whereas a Phe mutant still retained a significant degree of fusion activity. Liposome coflotation experiments showed that the fusion-negative Asp mutant did not form a stable interaction with membranes at low pH, although it was still capable of undergoing the structural rearrangements required for fusion. These data support the hypothesis that the cd loop may be directly involved in interactions with target membranes during fusion.

SUBMITTER: Allison SL

PROVIDER: S-EPMC114172 | biostudies-literature | 2001 May

REPOSITORIES: biostudies-literature

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Mutational evidence for an internal fusion peptide in flavivirus envelope protein E.

Allison S L SL Schalich J J Stiasny K K Mandl C W CW Heinz F X FX

Journal of virology 20010501 9

The envelope protein E of the flavivirus tick-borne encephalitis (TBE) virus promotes cell entry by inducing fusion of the viral membrane with an intracellular membrane after uptake by endocytosis. This protein differs from other well-studied viral and cellular fusion proteins because of its distinct molecular architecture and apparent lack of involvement of coiled coils in the low-pH-induced structural transitions that lead to fusion. A highly conserved loop (the cd loop), which resides at the ...[more]

PMID: 11287576

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Project description:Buffering of deleterious mutations by molecular chaperones and degradation of aberrant proteins by quality control systems are both major factors that can impact the mutational landscape available to a client protein. The impacts of the proteostasis network on protein evolution are not limited to just endogenous clients, but can also shape the mutational landscapes accessible to rapidly evolving viral proteins. Here, we test the hypothesis that the composition of the host cell’s endoplasmic reticulum (ER) proteostasis network shapes the evolution of RNA viruses by focusing on human immunodeficiency virus-1 envelope (Env), a membrane glycoprotein that folds and matures in the host cell’s secretory pathway. We apply chemical genetic methods to activate the IRE1-XBP1s and/or the ATF6 transcriptional arms of the unfolded protein response in a stress-independent manner. We then quantitatively assess the impact of the resulting altered host cell ER proteostasis environments on the relative enrichment of all Env single amino acid substitutions using deep mutational scanning. We find that upregulation of host ER proteostasis factors globally reduces the mutational tolerance of HIV-1 Env, particularly upon induction of the IRE1-XBP1s transcriptional arm of the UPR. The effects of ATF6 activation are less global, but still significant at particular Env sites. The impact of the XBP1s-induced ER proteostasis environment is disparate for diverse structural elements of Env. Conserved, functionally important regions generally exhibit the largest decreases in mutational tolerance upon XBP1s activation. In contrast, specific regions of Env, including regions targeted by broadly neutralizing antibodies, display greatly enhanced mutational tolerance when XBP1s is activated. Altogether, these data reveal a new set of host factors that specifically shape the mutational space accessible to HIV Env and, more generally, provide compelling evidence that UPR-regulated proteostasis mechanisms play critical roles in membrane protein evolution.

Dataset Information

Mutational evidence for an internal fusion peptide in flavivirus envelope protein E.

Publications

Mutational evidence for an internal fusion peptide in flavivirus envelope protein E.

Similar Datasets

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