IDENTIFICATION OF V3 LOOP-BINDING PROTEINS AS POTENTIAL RECEPTORS IMPLICATED IN THE BINDING OF HIV PARTICLES TO CD4(+) CELLS

The binding of human immunodeficiency virus (HIV) type 1 particles to CD4(+) cells could be blocked either by antibodies against the V3 loop domain of the viral external envelope glycoprotein gp120, or by the V 3 loop mimicking pseudopeptide 5[K psi(CH2N)PR]-TASP, which forms a st able complier with a cell-surface-expressed 95-kDa protein, Here, by u sing an affinity matrix containing 5[K psi(CH2N)PR]-TASP and cytoplasm ic extracts from human CEM cells, we purified three V3 loop-binding pr oteins of 95, 40, and 30 kDa, which after microsequencing were reveale d to be as nucleolin, putative HLA class II-associated protein (PHAP) II, and PHAP I, respectively. The 95-kDa cell-surface protein was also isolated and found to be nucleolin, We show that recombinant preparat ions of gp120 bind the purified preparations containing the V3 loop-bi nding proteins with a high affinity, comparable to the binding of gp12 0 to soluble CD4, Such binding is inhibited either by 5[K psi(CH2N)PR] -TASP or antibodies against the V3 loop. Moreover, these purified prep arations inhibit HIV entry into CD4+ cells as efficiently as soluble C D4, Taken together, our results suggest that nucleolin, PHAP II, and P HAP I appear to be functional as potential receptors in the HIV bindin g process by virtue of their capacity to interact with the V3 loop of gp120.