Cross- and Co-Packaging of Retroviral RNAs and Their Consequences

Abstract

Retroviruses belong to the family Retroviridae and are ribonucleoprotein (RNP) particles that contain a dimeric RNA genome. Retroviral particle assembly is a complex process, and how the virus is able to recognize and specifically capture the genomic RNA (gRNA) among millions of other cellular and spliced retroviral RNAs has been the subject of extensive investigation over the last two decades. The specificity towards RNA packaging requires higher order interactions of the retroviral gRNA with the structural Gag proteins. Moreover, several retroviruses have been shown to have the ability to cross-/co-package gRNA from other retroviruses, despite little sequence homology. This review will compare the determinants of gRNA encapsidation among different retroviruses, followed by an examination of our current understanding of the interaction between diverse viral genomes and heterologous proteins, leading to their cross-/co-packaging. Retroviruses are well-known serious animal and human pathogens, and such a cross-/co-packaging phenomenon could result in the generation of novel viral variants with unknown pathogenic potential. At the same time, however, an enhanced understanding of the molecular mechanisms involved in these specific interactions makes retroviruses an attractive target for anti-viral drugs, vaccines, and vectors for human gene therapy.

Biochemical and Functional Characterization of Mouse Mammary Tumor Virus Full-Length Pr77Gag Expressed in Prokaryotic and Eukaryotic Cells

“…Unlike the lentiviruses that are assembled at the plasma membrane, MMTV is a Betaretrovirus that displays a type B morphology [ 16 ] during replication where intracellular virus particles can be observed. Little is known about how the virus particle is assembled and in particular, the molecular mechanisms of MMTV genomic RNA (gRNA) packaging—a process that allows the virus to incorporate two copies of its single-stranded RNA genome into the assembling virus particle [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 ]. Retroviral RNA packaging requires specific interactions between both the gRNA and viral structural proteins, in particular Gag [ 25 , 26 , 27 , 28 ].…”

“…The NC serves as the key player in selective gRNA packaging. It is a highly basic protein containing zinc finger motifs to facilitate protein-RNA interactions [ 17 , 35 ]. By conducting mutational analysis, it has been established that the Gag NC domain of number of retroviruses is the most vital protein domain involved in the gRNA packaging process [ 36 , 37 , 38 , 39 , 40 ].…”

The bifurcated stem loop 4 (SL4) is crucial for efficient packaging of mouse mammary tumor virus (MMTV) genomic RNA

“…For example, a well-defined 5ʹ region of the murine leukemia virus (MLV) and some other retroviruses has been found to be required as well as sufficient for gRNA packaging, since inclusion of these sequences in heterologous non-retroviral RNAs facilitates the packaging of these ‘foreign’ RNAs into viral particles [14–17]. Furthermore, it is the presence of these structural motifs that explains the phenomena of co- and cross-packaging among retroviruses that have no relationship to each other [11]. For example, it has been long observed that specificity of packaging can be exchanged in many retroviruses by the substitution of packaging signals that have no sequence homology [16–22].…”

Role of Divalent Cations in HIV-1 Replication and Pathogenicity

“…The site of action of Zn 2+ is likely the HIV-1 NCp protein, a nucleic acid-binding protein composed of one or two cysteine-rich domains (CysteineX 2 CysteineX 4 HistidineX 4 Cysteine: CCHC motif) flanked by basic amino acids [136][137][138]. Moreover, NCp also regulates the trafficking of HIV-1 polyprotein Gag [139], HIV-1 genomic RNA (gRNA) dimerization [140,141], and generation of infectious progeny virion particles ( Figure 1) [142][143][144].…”