New insights into retrovirus encapsidation mechanisms 🦠

In an article published in Plos Biology, scientists show that the attachment of a retrovirus protein, called Gag, which plays a central role in the formation of viral particles, is necessary but not sufficient to enable the efficient incorporation of genomic RNA.

The Gag protein is essential for the assembly of retroviral particles...

The Gag protein plays a central role in the assembly of retroviruses.

On one hand, it serves as the precursor for the structural proteins of viral particles: around 2,000 copies of Gag assemble to form immature retroviral particles; after budding, the Gag molecules are cleaved, and the maturation products form the matrix, the capsid, and the nucleocapsid of mature infectious viral particles.

On the other hand, Gag selects the genomic RNA among the multitude of cellular or viral RNA species in the cytoplasm of infected cells, to ensure its encapsidation. To achieve this, Gag binds to a region of the genomic RNA that, depending on the retrovirus, is about 100 to 300 nucleotides long and contains encapsidation signals. This region is therefore called Psi (Packaging Signal). Psi is characterized, across all retroviruses, by the presence of long-range interactions (LRI) that maintain the overall structure of Psi.

... but its binding to the genome is not sufficient for encapsidation

While it was well-established that the binding of Gag to Psi is necessary for the selective encapsidation of the retroviral genome, it was unknown whether this binding alone could promote the process, and the role of long-range interactions in this process was poorly understood. To answer these questions, scientists used the murine mammary tumor virus (MMTV) as a model.

They identified mutations in a new long-range interaction that reduce genomic RNA encapsidation without affecting Gag’s affinity for Psi, thus demonstrating that Gag’s binding to Psi is necessary but not sufficient for the efficient incorporation of genomic RNA into retroviruses.

These mutations affect the overall structural organization of Psi without altering the local structure of the primary Gag binding sites that were previously identified in the wild-type virus by the same scientists. Nevertheless, they induce Gag to bind to other regions of Psi in the mutated viruses.

The results suggest that the three-dimensional structure of the complex formed between Gag and Psi regulates the assembly of viral particles around the genomic RNA, thus preventing the incorporation of other viral and cellular RNAs that also bind Gag with high affinity.

The genomic RNA packaging signals of MMTV contain a long-range interaction (LRI) identified in this study (upper part of the figure). In the wild-type virus, the Gag proteins (green ovals) bind to the PBS and ssPurines sites, leading to the encapsidation of the genomic RNA. Mutations that disrupt the long-range interaction (lower part of the figure) induce Gag binding to other sites, without loss of affinity. However, this binding does not allow the incorporation of the genomic RNA into viral particles.

References:

MMTV RNA packaging requires an extended long-range interaction for productive Gag binding to packaging signals.

Suresha G. Prabhu, Vineeta N. Pillai, Lizna Mohamed Ali, Valérie Vivet-Boudou, Akhil Chameettachal, Serena Bernacchi, Farah Mustafa, Roland Marquet, Tahir A. Rizvi. Published: October 3, 2024. Plos Biology. https://doi.org/10.1371/journal.pbio.3002827