Characterization of APOBEC3G binding to 7SL RNA

APOBEC proteins are members of a family of polynucleotide cytidine deaminases (CDA) that play important roles in antiviral defence. Human APOBEC3G and 3F can block the replication of a wide array of exogenous retroelements, including retroviruses such as human immunodeficiency virus (HIV) and murine leukaemia virus (MLV) [1,2], and hepadnaviruses such as hepatitis B virus (HBV) [3,4]. Primate lentiviruses including HIV counter APOBEC3G and 3F via their Vif protein, which binds to and triggers the proteasomal degradation of these cellular antivirals. In the absence of Vif, APOBEC3G and -F are packaged into retroviral particles, and lethally edit nascent viral reverse transcripts [1,2,5,6]. What tethers APOBEC proteins to virions has so far remained incompletely characterized. While some have invoked a role for the viral genomic RNA, more undisputed is the claimed importance of the nucleocapsid region of HIV-1 Gag and of yet unknown cellular RNAs in this process [7-14].APOBEC family members can also act on endogenous substrates, notably retroelements. APOBEC3A, 3G and 3F can block the propagation of endogenous retroviruses such as intracisternal-A particles (IAP) [15,16] or MusD, and APOBEC3A, 3B and, to a lesser extent, 3C and 3F can inhibit LINE-1 (Long Interspersed Nuclear Element 1) retrotransposition [15,17-20]. Furthermore, APOBEC3A, APOBEC3B, APOBEC3C and APOBEC3G can prevent Alu retrotransposition, a process mediated in trans by the reverse transcriptase and integrase activities encoded by LINE [17,18]. Interestingly, APOBEC3G overexpression appears to recruit Alu RNAs into APOBEC3G-containing high molecular mass ribonucleoprotein complexes [21].The Alu family of repetitive sequences is one of the most successful groups of mobile genetic elements, having multiplied to more than one million copies in the human genome in some 65 million years of primate evolution [22,23]. Interestingly, the emergence of Alu as major primate genome remodelers has coincided with t