THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 TRANSMEMBRANE GP41 PROTEIN ISA CALCIUM-BINDING PROTEIN AND INTERACTS WITH THE PUTATIVE 2ND-RECEPTOR MOLECULES IN A CALCIUM-DEPENDENT MANNER

Fusion is a crucial event in the life cycle of the human immunodeficie ncy virus (HIV); it is initiated by the high-affinity binding between gp120, the external surface glycoprotein of HIV-1, and the differentia tion antigen CD4 and finally results in the insertion of the hydrophob ic amino terminus of the gp41 envelope glycoprotein into the plasma me mbrane of the target cell, Recent results suggest that this process is dependent upon calcium ions, but the mechanism or the proteins involv ed are not understood, Computer assisted sequence analysis revealed a putative calcium binding site within the extracellular part of gp41 th at was highly reminiscent of the calcium-binding EF-hand structure, To test this hypothesis, calcium-binding experiments were performed, Bin ding of a recombinant soluble form of the transmembrane protein (rsgp4 1) to its putative second-receptor molecules in equilibrium was depend ent 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 bo vine serum albumin, Binding was inhibited by the addition of nonradioa ctive calcium, indicating that binding was specific. Neither magnesium nor manganese inhibited the binding of labeled calcium to rsgp41, Bin ding was dependent on the oxidative state of the rsgp41 molecule, sugg esting the functional importance of the correctly folded structure of the rsgp41 protein, In this report, we demonstrate that the HIV-1 tran smembrane protein gp41 is a calcium-binding protein and interacts with the putative second-receptor molecules in a calcium-dependent manner.