Isolation and Characterization of a Novel Protein (X-ORF Product) from SIV and HIV-2

A protein designated p14 was purified from a simian immunodeficiency virus (SIV$_{\text{Mne}}$) and was shown by amino acid sequence analysis to be nearly identical to the predicted translational product of a unique open reading frame (X-ORF) in the nucleotide sequences of SIV$_{\text{mac}}$ and human immunodeficiency virus type 2 (HIV-2). Thus the X-ORF is proven to be a new retroviral gene. The p14 is present in SIV$_{\text{Mne}}$ in molar amounts equivalent to those of the gag proteins. This is the first example of a retrovirus that contains a substantial quantity of a viral protein that is not a product of the gag, pro, pol, or env genes. SIV p14 and its homolog in HIV-2 may function as nucleic acid binding proteins since purified p14 binds to single-stranded nucleic acids in vitro. Antisera to the purified protein detected p14 in SIV$_{\text{Mne}}$, SIV$_{\text{mac}}$, and a homologous protein (16 kilodaltons) in HIV-2 but did not react with HIV-1. Diagnostic procedures based on this novel protein will distinguish between HIV-1 and HIV-2.