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

While several cellular proteins are incorporated in the human immunodeficie ncy 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 th e N-terminal region of the capsid (CA) domain of the precursor polyprotein, Pr55(Gag). Even when prevented from interacting with CyP A, Pr55(Gag) stil l forms particles that proceed to mature into morphologically wild-type vir ions, 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 (W-121) in CyP A distinguished the two p roteins: a phenylalanine substitution (W121F) impaired binding of mature CA protein but not of Gag. This indicates the occurrence of a maturation-depe ndent switch in the conformation of the Pro-rich loop. A structural consequ ence of Gag binding to CyP A was to block this maturational refolding, resu lting in a 24-kDa CA protein retaining the immature Pro-rich loop conformat ion. Using trypsin as a structure probe, we demonstrate that the conformati on 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 indicat ed by a reduction in the accessibility of Cys residue(s) in the region to c hemical modification. Hence, the end result: of binding to CyP A, whether t he Pro-rich loop is in the context of Gag or mature CA protein, is a struct urally modified mature CA protein. The postassembly role of CyP A may be me diated through these modified mature CA proteins.