Structural consequences of cyclophilin a binding on maturational refolding in human immunodeficiency virus type 1 capsid protein

While several cellular proteins are incorporated in the human immunodeficiency virus type 1 virion, cyclophilin (CyP) A is the only one whose absence has been demonstrated to impair infectivity. Incorporation of the cytosolic protein results from interaction with a highly exposed Pro-rich loop in the N-terminal region of the capsid (CA) domain of the precursor polyprotein, Pr55^Gag. Even when prevented from interacting with CyP A, Pr55^Gag still forms particles that proceed to mature into morphologically wild-type virions, suggesting that CyP A influences a postassembly event. The nature of this CyP A influence has yet to be elucidated. Here, we show that while CyP A binds both Gag and mature CA proteins, the two binding interactions are actually different. Tryptophan 121 (W₁₂₁) in CyP A distinguished the two proteins: a phenylalanine substitution (W₁₂₁F) impaired binding of mature CA protein but not of Gag. This indicates the occurrence of a maturation-dependent switch in the conformation of the Pro-rich loop. A structural consequence of Gag binding to CyP A was to block this maturational refolding, resulting in a 24-kDa CA protein retaining the immature Pro-rich loop conformation. Using trypsin as a structure probe, we demonstrate that the conformation of the C-terminal region in mature CA is also a product of maturational refolding. Binding to wild-type CyP A altered this conformation, as indicated by a reduction in the accessibility of Cys residue(s) in the region to chemical modification. Hence, the end result of binding to CyP A, whether the Pro-rich loop is in the context of Gag or mature CA protein, is a structurally modified mature CA protein. The postassembly role of CyP A may be mediated through these modified mature CA proteins.