Human APOBEC3 proteins can inhibit xenotropic murine leukemia virus-related virus infectivity

AbstractXenotropic murine leukemia virus-related virus (XMRV) is a novel retrovirus, related to murine leukemia virus (MLV), that has been implicated in human disease. If XMRV is indeed able to replicate in humans, then one might predict that XMRV would have developed resistance to human innate antiviral resistance factors such as APOBEC3G (hA3G). In fact, we observed that XMRV and MLV are both highly sensitive to inhibition by hA3G and equally resistant to inhibition by murine APOBEC3. While several human prostate cancer cell lines were found to lack hA3G, stable expression of physiological levels of hA3G rendered these cells refractory to XMRV replication. Few human tissues fail to express hA3G, and we therefore hypothesize that XMRV replicates in one or more hA3G-negative reservoir tissues and/or that human XMRV infections are likely to be rare and potentially of zoonotic origin

Single-stranded RNA facilitates nucleocapsid: APOBEC3G complex formation

Binding of APOBEC3G to the nucleocapsid (NC) domain of the human immunodeficiency virus (HIV) Gag polyprotein may represent a critical early step in the selective packaging of this antiretroviral factor into HIV virions. Previously, we and others have reported that this interaction is mediated by RNA. Here, we demonstrate that RNA binding by APOBEC3G is key for initiation of APOBEC3G:NC complex formation in vitro. By adding back nucleic acids to purified, RNase-treated APOBEC3G and NC protein preparations in vitro, we demonstrate that complex formation is rescued by short (≥10 nucleotides) single-stranded RNAs (ssRNAs) containing G residues. In contrast, complex formation is not induced by add-back of short ssRNAs lacking G, by dsRNAs, by ssDNAs, by dsDNAs or by DNA:RNA hybrid molecules. While some highly structured RNA molecules, i.e., tRNAs and rRNAs, failed to rescue APOBEC3G:NC complex formation, other structured RNAs, i.e., human Y RNAs and 7SL RNA, did promote NC binding by APOBEC3G. Together, these results indicate that ternary complex formation requires ssRNA, but suggest this can be presented in the context of an otherwise highly structured RNA molecule. Given previous data arguing that APOBEC3G binds, and edits, ssDNA effectively in vitro, these data may also suggest that APOBEC3G can exist in two different conformational states, with different activities, depending on whether it is bound to ssRNA or ssDNA

Identification of a novel cellular cofactor for the Rev/Rex class of retroviral regulatory proteins

AbstractHIV-1 Rev is the prototype of a class of retroviral regulatory proteins that induce the sequence-specific nuclear export of target RNAs. This function requires the Rev activation domain, which is believed to bind an essential cellular cofactor. We report the identification of a novel human gene product that binds to not only the HIV-1 Rev activation domain in vitro and in vivo but also to functionally equivalent domains in other Rev and Rex proteins. The Rev/Rex activation domain-binding (Rab) protein occupies a binding site on HIV-1 Rev that precisely matches that predicted by genetic analysis. Rab binds the Rev activation domain when Rev is assembled onto its RNA target and can significantly enhance Rev activity when overexpressed. We conclude that Rab is the predicted activation domain-specific cofactor for the Rev/Rex class of RNA export factors