title: Topology of the disulfide bonds in the antiviral lectin scytovirin.
abstract: The antiviral lectin scytovirin (SVN) contains a total of five disulfide bonds in two structurally similar domains. Previous reports provided contradictory results on the disulfide pairing in each individual domain, and we have now re-examined the disulfide topology. N-terminal sequencing and mass spectrometry were used to analyze proteolytic fragments of native SVN obtained at acidic pH, yielding the assignment as Cys7-Cys55, Cys20-Cys32, Cys26-Cys38, Cys68-Cys80, and Cys74-Cys86. We also analyzed the N-terminal domain of SVN (SD1, residues 1-48) prepared by expression/oxidative folding of the recombinant protein and by chemical synthesis. The disulfide pairing in the chemically synthesized SD1 was forced into predetermined topologies: SD1A (Cys20-Cys26, Cys32-Cys38) or SD1B (Cys20-Cys32, Cys26-Cys38). The topology of native SVN was found to be in agreement with the SD1B and the one determined for the recombinant SD1 domain. Although the two synthetic forms of SD1 were distinct when subjected to chromatography, their antiviral properties were indistinguishable, having low nM activity against HIV. Tryptic fragments, the "cystine clusters" [Cys20-Cys32/Cys26-Cys38; SD1] and [Cys68-Cys80/Cys74-C-86; SD2], were found to undergo rapid disulfide interchange at pH 8. This interchange resulted in accumulation of artifactual fragments in alkaline pH digests that are structurally unrelated to the original topology, providing a rational explanation for the differences between the topology reported herein and the one reported earlier (Bokesh et al., Biochemistry 2003;42:2578-2584). Our observations emphasize the fact that proteins such as SVN, with disulfide bonds in close proximity, require considerable precautions when being fragmented for the purpose of disulfide assignment.
authors.affiliation: Protein Structure Section, Macromolecular Crystallography Laboratory, National Cancer Institute, NCI-Frederick, Frederick, Maryland 21702-1201, USA.
authors.forename: Tinoush
authors.initials: T
authors.lastname: Moulaei
authors.forename: Olga
authors.initials: O
authors.lastname: Stuchlik
authors.forename: Matthew
authors.initials: M
authors.lastname: Reed
authors.forename: Weirong
authors.initials: W
authors.lastname: Yuan
authors.forename: Jan
authors.initials: J
authors.lastname: Pohl
authors.forename: Wuyuan
authors.initials: W
authors.lastname: Lu
authors.forename: Lauren
authors.initials: L
authors.lastname: Haugh-Krumpe
authors.forename: Barry R
authors.initials: BR
authors.lastname: O'Keefe
authors.forename: Alexander
authors.initials: A
authors.lastname: Wlodawer
mesh_terms: D000595:Amino Acid Sequence; D000998:Antiviral Agents / Q000737:chemistry* / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000458:Cyanobacteria / Q000737:chemistry* / Q000378:metabolism; D003553:Cystine / Q000378:metabolism; D004220:Disulfides / Q000032:analysis*; D037102:Lectins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D010434:Pepsin A / Q000378:metabolism; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D000998:Antiviral Agents; D001426:Bacterial Proteins; D002352:Carrier Proteins; D004220:Disulfides; D037102:Lectins; D008565:Membrane Proteins; D011994:Recombinant Proteins; C474342:scytovirin protein, S varium; D003553:Cystine; D010434:Pepsin A