Cyanovirin-N, a potent human immunodeficiency virus-inactivating protein, blocks both CD4-dependent and CD4-independent binding of soluble gp120 (sgp120) to target cells, inhibits sCD4-induced binding of sgp120 to cell-associated CXCR4, and dissociates bound sgp120 from target cells.

Cyanovirin-N (CV-N), an 11-kDa protein originally isolated from the cyanobacterium Nostoc ellipsosporum, potently inactivates diverse strains of human immunodeficiency virus type 1 (HIV-1), HIV-2, simian immunodeficiency virus, and feline immunodeficiency virus. It has been well established that the HIV surface envelope glycoprotein gp120 is a molecular target of CV-N. We recently reported that CV-N impaired the binding of virion-associated gp120 to cell-associated CD4 and that CV-N preferentially inhibited binding of the glycosylation-dependent neutralizing monoclonal antibody 2G12 to gp120. However, CV-N did not interfere with the interactions of soluble CD4 (sCD4) with either soluble gp120 (sgp120) or virion-associated gp120. In the present study, we have evaluated the effects of CV-N on the binding of sgp120 to cell-associated CD4 to clarify the experimental basis of the previous binding results, and we further address the detailed mechanism of action of CV-N. Here we present evidence that (i) CV-N impairs both CD4-dependent and CD4-independent binding of sgp120 to the target cells, (ii) CV-N blocks the sCD4-induced binding of sgp120 with cell-associated coreceptor CXCR4, and (iii) CV-N dissociates bound sgp120 from target cells. The results illustrate that the measured effects of CV-N on gp120-CD4 binding interactions depend upon the type of CD4 (soluble or cell associated), but not upon the type of gp120 (soluble or virion associated), employed in the experimental protocol. In addition, this study reinforces that CV-N acts uniquely to prevent essential interactions between the envelope glycoprotein and target cell receptors and further supports the potential broad utility of CV-N as a microbicide to prevent the transmission of HIV and AIDS.