Calcium ions are required for cell fusion mediated by the CD4-human immunodeficiency virus type 1 envelope glycoprotein interaction.
Calcium ions are required for fusion of a wide variety of artificial and biological membranes. To examine the role of calcium ions for cell fusion mediated by interactions between CD4 and the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (gp120-gp41), we used two experimental systems: (i) cells expressing gp120-gp41 and its receptor CD4, both encoded by recombinant vaccinia viruses, and (ii) chronically infected cells producing low levels of HIV-1. Fusion was measured by counting the number of syncytia and by monitoring the redistribution of fluorescence dyes by video microscopy. Syncytia did not form in solutions without calcium ions. Addition of calcium ions partially restored the formation of syncytia. EDTA and EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] blocked syncytium formation in culture media containing calcium ions. Membrane fusion as monitored by fluorescence dye redistribution also required calcium ions. Cell fusion increased with an increase in calcium ion concentration from 100 microM to 10 mM but was not affected by magnesium ions in the concentration range from 0 to 30 mM. Fibrinogen and fibronectin did not promote fusion in the absence or presence of Ca2+. Binding of soluble CD4 to gp120-gp41-expressing cells was not affected by Ca2+ and Mg2+. We conclude that Ca2+ is involved in postbinding steps in cell fusion mediated by the CD4-HIV-1 envelope glycoprotein interaction.
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Section on Membrane Structure and Function, National Cancer Institute, Bethesda, Maryland 20892.
Broder
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Berger
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Blumenthal
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D000163:Acquired Immunodeficiency Syndrome / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D001386:Azides / Q000494:pharmacology; D015704:CD4 Antigens / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D002459:Cell Fusion / Q000187:drug effects*; D003513:Cycloheximide / Q000494:pharmacology; D015699:HIV Envelope Protein gp120 / Q000378:metabolism; D015700:HIV Envelope Protein gp41 / Q000378:metabolism; D015497:HIV-1 / Q000378:metabolism*; D008274:Magnesium / Q000494:pharmacology; D008561:Membrane Fusion; D019810:Sodium Azide; D014759:Viral Envelope Proteins / Q000378:metabolism*
D001386:Azides; D015704:CD4 Antigens; D015699:HIV Envelope Protein gp120; D015700:HIV Envelope Protein gp41; D014759:Viral Envelope Proteins; D000255:Adenosine Triphosphate; D019810:Sodium Azide; D003513:Cycloheximide; D008274:Magnesium; D002118:Calcium