# Encapsidation of Staufen-2 Enhances Infectivity of HIV-1
## 1 Introduction  
HIV-1 relies on host cellular machinery for its infectivity and propagation. HIV-1 RNA associates with many host RNA binding proteins (RBPs) during its transport and translation. The molecular composition of HIV-1 RNA-associated RBP containing ribonucleoprotein (RNP) complexes undergo dynamic changes during virus propagation, associating with distinct host proteins at different stages, such as splicing, nuclear export of viral RNA, translation, assembly, and budding [1; 2; 3]. The cumulative impact of these interactions determines HIV-1 production and viral population dynamics.  
HIV-1 Gag plays a crucial role in viral RNA encapsidation and particle assembly. During this process, HIV-1 Gag associates with several host RBPs forming RNP complexes. The host proteins bound to HIV-1 Gag and viral RNA in RNPs are often packed into new virus particles [4; 5]. Gag-interacting host factors can either support or restrict viral replication. One such example is a restriction factor APOBECC3G (A3G), which interacts with the nucleocapsid (NC) domain of HIV-1 Gag and gets packaged into progeny virions in the producer cells in the absence of HIV-1 Vif protein that degrades A3G. During the next round of infection, the incorporated A3G binds to the minus strand of the proviral DNA andgenerates mutations by cytidine deamination, thereby inhibiting HIV-1 [6; 7; 8; 9; 10]. Similarly, Staufen-1 interacts with the NC domain of HIV-1 Gag and is incorporated into emerging virus particles, usually acting as a positive regulator of HIV-1 propagation [11; 12; 13; 14].  
Staufen is a family of RBPs that affects RNA localization in many organisms [15]. First identified in Drosophila, Staufen proteins were later found to be ubiquitously expressed in all mammalian cells and are known to play diverse roles in mRNA transport, translation, and mRNA decay [15; 16; 17]. Mammalian cells express two Staufen paralogs termed Staufen-1 and Staufen-2; both proteins are derived from separate genes and produce several isoforms via pre-mRNA alternative splicing or polyadenylation. Both Staufen-1 and Staufen-2 contain four double-stranded RNA-binding domains (dsRBD) and bind to distinct but overlapping sets of mRNAs [18; 19]. Although several studies define multiple roles of Staufens in neural cells [20; 21; 22; 23], these proteins are also expressed across different cell types, including T cells and macrophages, and are involved in different functions, such as cell-cycle regulation, apoptosis, organogenesis, and regulation of inflammatory and immune response genes expression [18; 24; 25; 26; 27; 28; 29].  
While both Staufen proteins have been studied extensively for their roles in regulating the post-transcriptional activity of RNA transport, translation, and decay, Staufen-1 has been recognized as a positive effector of several RNA viruses, such as enterovirus [30], influenza virus [31], hepatitis C virus [32], HCV [33], endogenous HERV-K (HML-2) [34], and HIV-1 [12; 35]. Staufen-1 interacts with HIV-1 Gag and HIV-1 RNA to facilitate various activities such as RNA transport and packaging [35; 36]. Like Staufen-1, Staufen-2 aids in the trafficking of RNP granules in neurons and is actively involved in mRNA transport and local translation [37; 38; 39]. Interaction of Staufen-1 and Staufen-2 with HIV-1 RNA was reported to enhance viral RNA encapsidation [14]. Aside from interacting with HIV-1 RNA, our previous study established that Staufen-2 interacts with HIV-1 protein Rev and increases Rev-associated viral RNA export from the nucleus to the cytoplasm [40]. A similar association of Staufen-1 with HIV-1 Rev is not reported, although Staufen-1 has been shown to interact with Rec of HERV-K (HML-2) [34]. Interestingly, Staufen-1 and Staufen-2 are reported to co-localize in cells and occur as typical components of RNP granules [18; 39]. Considering the involvement of Staufen-1 in supporting HIV-1 infection, we set out to investigate if Staufen-2 gets encapsidated into virus particles and affects viral infectivity.