CYCLOSPORINE-A-RESISTANT HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 MUTANTS DEMONSTRATE THAT GAG ENCODES THE FUNCTIONAL TARGET OF CYCLOPHILIN-A

The cellular peptidyl-prolyl isomerase cyclophilin A is incorporated i nto human immunodeficiency virus type 1 virions via contacts with the proline-rich domain of the Gag polyprotein. Cyclosporine A and nonimmu nosuppressive analogs bind with high affinity to cyclophilin A, compet e with Gag for binding to cyclophilin A, and prevent incorporation of cyclophilin A into virions; in parallel with the disruption of cycloph ilin A incorporation into virions, there is a linear reduction in the initiation of reverse transcription after infection of a T cell. Passa ge of human immunodeficiency virus type 1 in the presence of the drug selects one of two mutations, either of which alters the proline-rich domain of Gag and is sufficient to confer drug resistance on the clone d wild-type provirus. Neither mutation alters Gag's cyclophilin A-bind ing properties in vitro, and cyclophilin A incorporation into drug-res istant virions is effectively disrupted by cyclosporine A, indicating that the drug-resistant mutants do not require virion-associated cyclo philin A to initiate infection. That Gag's functional dependence on cy clophilin ii can be differentiated genetically from its ability to bin d cyclophilin A is further demonstrated by the rescue of a mutation pr ecluding cyclophilin A packaging by a mutation conferring cyclosporine A resistance. These experiments demonstrate that, in addition to its ability to package cyclophilin A into virions, gag encodes the functio nal target of cyclophilin A.