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J. Virol., 08 1996, 5541-5547, Vol 70, No. 8
M Carleton and DT Brown
The Sindbis virus envelope is composed of 80 E1-E2 (envelope glycoprotein)
heterotrimers organized into an icosahedral protein lattice with T=4
symmetry. The structural integrity of the envelope protein lattice is
maintained by E1-E1 interactions which are stabilized by intramolecular
disulfide bonds. Structural domains of the envelope proteins sustain the
envelope's icosahedral lattice, while functional domains are responsible
for virus attachment and membrane fusion. We have previously shown that
within the mature Sindbis virus particle, the structural domains of the
envelope proteins are significantly more resistant to the
membrane-permeative, sulfhydryl- reducing agent dithiothreitol (DTT) than
are the functional domains (R. P. Anthony, A. M. Paredes, and D. T. Brown,
Virology 190:330-336, 1992). We have used DTT to probe the accessibility of
intramolecular disulfides within PE2 (the precursor to E2) and E1, as these
proteins fold and are assembled into the spike heterotrimer. We have
determined through pulse-chase analysis that intramolecular disulfide bonds
within PE2 are always sensitive to DTT when the glycoproteins are in the
endoplasmic reticulum. The reduction of these disulfides results in the
disruption of PE2-E1 associations. E1 acquires increased resistance to DTT
as it folds through a series of disulfide intermediates (E1alpha, - beta,
and -gamma) prior to assuming its native and most compact conformation
(E1epsilon). The transition from a DTT-sensitive form into a form which
exhibits increased resistance to DTT occurs after E1 has folded into its
E1beta conformation and correlates temporally with the dissociation of
BiP-E1 complexes and the formation of PE2-E1 heterotrimers. We propose that
the disulfide bonds within E1 which stabilize the protein domains required
for maintaining the structural integrity of the envelope protein lattice
form early within the folding pathway of E1 and become inaccessible to DTT
once the heterotrimer has formed.
Copyright © 1996, American Society for Microbiology
Disulfide bridge-mediated folding of Sindbis virus glycoproteins
The Cell Research Institute and Department of Microbiology, University of Texas at Austin, 78713-7640, USA.
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