Role of the Capsid Helix 4-5 Loop in Equine Infectious Anemia Virus Infection

The lentiviral capsid core, which encapsulates the viral RNA genome, is delivered into the target cell cytoplasm during the viral entry process. In the cytoplasm, the conical core undergoes morphological changes, which are termed uncoating. Proper uncoating has been shown to be critical for the infectivity of the lentivirus HIV-1. In addition, the HIV-1 capsid protein is critical for the process of nuclear import of the preintegration complex (PIC). The lentivirus equine infectious anemia virus (EIAV) shares many similarities to HIV-1, including similarities in the capsid protein. In particular, both HIV-1 and EIAV capsid contain a proline-rich loop region in the amino terminal domain of capsid between helices 4 and 5. The host cellular factor cyclophilin A binds this loop in HIV-1 and is critical for proper uncoating. We hypothesized that this helix 4-5 loop was also critical for EIAV infectivity at some early step in the viral infection cycle. We created a panel of amino acid substitution mutations in this loop region. Some of the mutations resulted in severely deleterious effects on EIAV infectivity. Some mutations caused a slight increase in infectivity. The deleterious mutations did not affect uncoating or reverse transcription but appeared to block nuclear import of the PIC. Those mutations in which infectivity was slightly increased did not exhibit significantly different phenotypes from wild-type EIAV at any of the stages examined. The results of this study lend further support to the role of capsid as a determinant of nuclear import and suggest that viral and cellular factors critical to HIV-1 import may also be applicable to EIAV. Future research should focus on identifying the causes of the defects in nuclear import observed for some mutants, as well as attempt to identify the reason for the infectivity increase in others. In addition, inclusion of EIAV in future studies of nuclear import involving HIV-1 can broaden the scope of the data to lentiviruses in general rather than HIV-1 in particular.