Clearly different mechanisms of enhancement of short-lived Nef-mediated viral infectivity between SIV and HIV-1

BACKGROUND: One of the major functions of Nef is in the enhancement of the infectivity of the human and simian immunodeficiency viruses (HIV and SIV, respectively). However, the detailed mechanism of the enhancement of viral infectivity by Nef remains unclear. Additionally, studies of mechanisms by which Nef enhances the infectivity of SIV are not as intensive as those of HIV-1. METHODS: We generated short-lived Nef constructed by fusing Nef to a proteasome-mediated protein degradation sequence to characterize the Nef role in viral infectivity. RESULTS: The apparent expression level of the short-lived Nef was found to be extremely lower than that of the wild-type Nef. Moreover, the expression level of the short-lived Nef increased with the treatment with a proteasome inhibitor. The infectivity of HIV-1 with the short-lived Nef was significantly lower than that with the wild-type Nef. On the other hand, the short-lived Nef enhanced the infectivity of SIV(mac239), an ability observed to be interestingly equivalent to that of the wild-type Nef. The short-lived Nef was not detected in SIV(mac239), but the wild-type Nef was, suggesting that the incorporation of Nef into SIV(mac239) is not important for the enhancement of SIV(mac239) infectivity. CONCLUSIONS: Altogether, the findings suggest that the mechanisms of infectivity enhancement by Nef are different between HIV-1 and SIV(mac239). Lastly, we propose the following hypothesis: even when the expression level of a protein is extremely low, the protein may still be sufficiently functional.