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Isomerization of the intersubunit disulphide-bond in Env controls retrovirus fusion

Michael Wallin1, Maria Ekström, Henrik Garoff

  • 1Department of Biosciences at Novum, Karolinska Institute, Huddinge, Sweden.

The EMBO Journal
|December 20, 2003
PubMed

Insights

Murine leukemia virus Env fusion requires calcium ions (Ca2+). Receptor binding triggers Ca2+ removal, leading to SU-TM complex dissociation and virus fusion activation.

Area of Science:

  • Virology
  • Molecular Biology
  • Biochemistry

Background:

  • The envelope (Env) protein of murine leukemia virus mediates membrane fusion.
  • Env consists of surface (SU) and transmembrane (TM) subunits, crucial for viral entry.

Purpose of the Study:

  • To elucidate the role of calcium ions (Ca2+) and disulfide bonds in the Env-mediated membrane fusion process.
  • To understand the mechanism of Env activation upon receptor binding.

Main Methods:

  • Disulfide bond disruption assays using various chemical treatments (NP-40, heat, urea) and Ca2+ depletion.
  • Thiol mapping to identify accessible thiol groups and disulfide bond isomerization.
  • Alkylation studies to probe the function of specific thiol groups.
  • Dithiothreitol (DTT) treatment to cleave disulfide bonds.
  • Calcium ion concentration-dependent fusion assays.

Main Results:

  • Env subunits form disulfide-linked SU-TM complexes, disrupted by Ca2+ depletion, heat, urea, or NP-40.
  • Ca2+ depletion or receptor binding exposes a thiol group in the SU subunit's Cys-X-X-Cys (CXXC) motif, inducing disulfide bond isomerization and SU-TM dissociation.
  • Alkylation of this thiol group inhibits fusion and infection, while DTT treatment rescues fusion.
  • High Ca2+ concentrations inhibit fusion, whereas low concentrations enhance it.

Conclusions:

  • Calcium ions (Ca2+) stabilize the Env structure.
  • Receptor binding initiates a cascade: Ca2+ removal, thiol exposure, disulfide bond isomerization, SU dissociation, and ultimately, fusion activation.

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