Abstract
Fusion is a crucial event in the life cycle of the human immunodeficiency virus (HIV); is is initiated by the high-affinity binding between gp120, the external surface glycoprotein of HIV-1, and the differentiation antigen CD4 and finally results in the insertion of the hydrophobic amino terminus of the gp41 envelope glycoprotein into the plasma membrane of the target cell. Recent results suggest that this process is dependent upon calcium ions, but the mechanism or the proteins involved are not understood. Computer-assisted sequence analysis revealed a putative calcium-binding site within the extracellular part of gp41 that was highly reminiscent of the calcium-binding EF-hand structure. To test this hypothesis, calcium-binding experiments were performed. Binding of a recombinant soluble form of the transmembrane protein (rsgp41) to its putative second-receptor molecules in equilibrium was dependent upon calcium. The affinity was not influenced by calcium, but the maximum binding was increased in a dose-dependent manner. Radioactive calcium bound to rsgp41 covalently attached to Sepharose but not to bovine serum albumin. Binding was inhibited by the addition of nonradioactive calcium, indicating that binding was specific. Neither magnesium nor manganese inhibited the binding of labeled calcium to rsgp41. Binding was dependent on the oxidative state of the rsgp41 molecule, suggesting the functional importance of the correctly folded structure of the rsgp41 protein. In this report, we demonstrate that the HIV-1 transmembrane protein gp41 is a calcium-binding protein and interacts with the putative second-receptor molecules in a calcium-dependent manner.