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Journal of Virology, December 2001, p. 11881-11885, Vol. 75, No. 23
Department of Molecular Sciences, University
of Tennessee Health Sciences Center, Memphis, Tennessee 38163
Received 3 July 2000/Accepted 5 September 2001
Two second-site mutations in Moloney murine leukemia virus envelope
surface protein (SU) were previously shown to rescue infection of two
different SU mutants, a fusion-defective point mutant and a
fusion-defective modified SU that exhibits weak subunit association. We
report here that they also rescue infection of a third defective SU,
one modified by insertion of the green fluorescent protein (GFP)
between serine 6 and proline 7. GFP-SU assembled into virions and
showed a strong association with the transmembrane protein (TM).
However, these virions were noninfectious. GFP-SU expression was not
maintained within cells, suggesting that the protein was toxic.
Addition of the second-site mutations rendered the GFP-SU virus
infectious and resulted in prolonged expression of the modified envelope protein. This virus showed a slight reduction in receptor binding but not in envelope protein processing, suggesting that addition of the GFP sequences results in subtle structural changes. Extrapolating these data, we see that the fundamental problem with the
GFP-SU envelope protein appears to be a folding problem, suggesting
that the second-site mutations rescue GFP-SU primarily by a mechanism
that involves stabilizing the envelope protein structure.
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.23.11881-11885.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Two Point Mutations Produce Infectious Retrovirus
Bearing a Green Fluorescent Protein-SU Fusion Protein

and
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Corresponding author. Mailing address: Department of
Molecular Sciences, University of Tennessee Health Sciences Center, 858 Madison Ave., Rm. G01, Memphis, TN 38163. Phone: (901) 448-5521. Fax:
(901) 448-7360. E-mail: lalbritton{at}utmem.edu.
Present address: Department of Cell Biology, Duke University
Medical Center and Howard Hughes Medical Institute, Durham, NC 27710.
Present address: Program in Molecular Medicine, University of
Massachusetts Medical School, Worcester, MA 01605.
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