Structural features of CD4 required for binding to HIV.

A soluble form of the human CD4 glycoprotein (sCD4), the cellular receptor for human HIV, was treated with various physical, chemical, and enzymic regimens and tested over a range of concentrations for its capacity to inhibit the binding of HIV to CD4+ T cells. Reduction of disulfide bonds and alkylation in denaturing buffer (8 M urea) destroyed the inhibitory activity of sCD4, whereas reduction and alkylation in PBS had no effect. Derivatization or digestion of carbohydrate groups by periodate oxidation or by glycolytic enzyme digestion did not affect sCD4 inhibitory capacity. Digestion with trypsin or endoproteinase Glu-C destroyed activity. A limited digestion of sCD4 with endoproteinase Glu-C resulted in a mixture of fragments, however, and the mixture had inhibitory activity equivalent to that of intact sCD4. Within this mixture, a fragment of 23 kDa was identified that binds to HIV. Although sCD4 can be digested to yield fully active fragments, the requirement for intrachain disulfide bonding indicates that the minimum sized portion of CD4 that will retain full affinity for HIV will have to be formulated with a proper tertiary structure.