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Journal of Virology, January 2000, p. 218-227, Vol. 74, No. 1
School of Pharmacy1
and Department of Electron Microscopy,3
University of Oslo, 0316 Oslo, and National Veterinary
Institute, 0033 Oslo,2 Norway
Received 26 May 1999/Accepted 29 September 1999
We have investigated the initial steps in the interaction between
infectious salmon anemia virus (ISAV) and cultured cells from Atlantic
salmon (SHK-1 cell line). Using radioactively or fluorescently labelled
viral particles we have studied the binding and fusion
kinetics and the effect of pH on binding, uptake, and fusion of ISAV to
SHK-1 cells and liposomes. As pH in the medium was reduced from 7.5 to
4.5, the association of virus to the cells was nearly doubled. The same
effect of pH was observed when fusion between ISAV and liposomes was
analyzed. In addition, the binding of ISAV to intact SHK-1 cells and to
cell membrane proteins blotted onto filters was neuraminidase
sensitive. However, the increased binding induced by low pH was not
neuraminidase sensitive, probably reflecting activation of a fusion
peptide at low pH. By using confocal fluorescence microscopy, the
increased fusion of fluorescently labelled ISAV with the plasma
membrane due to low pH could be demonstrated. When
vacuolar pH in the cells was raised during inoculation with chloroquine
or ammonium chloride, both electron and confocal microscopy
showed accumulation of ISAV in endosomes and lysosomes. Production of
infectious virus could be increased by lowering the extracellular pH
during infection. Furthermore, chloroquine present during virus
inoculation also caused a reduction in the synthesis of viral proteins
in ISAV-infected cells as well as in the production of infective virus.
These results indicate that ISAV binds to sialic acid residues on the
cell surface and that the fusion between virus and cell membrane takes
place in the acid environment of endosomes. This provides
further evidence for a high degree of similarity between ISAV and
influenza virus and extends the basis for the classification of
this virus as a member of the Orthomyxoviridae family.
0022-538X/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Initial Events in Infectious Salmon Anemia Virus
Infection: Evidence for the Requirement of a Low-pH Step
*
Corresponding author. Mailing address: School of
Pharmacy, University of Oslo, P.O. Box 1068 Blindern, 0316 Oslo,
Norway. Phone: 4722844943. Fax: 4722844944. E-mail:
tor.gjoen{at}embnet.uio.no.
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