African swine fever virus transmembrane protein pEP84R guides core assembly

Abstract

African swine fever virus (ASFV) causes a devastating hemorrhagic disease with worldwide circulation and no widely available therapeutic prevention. The infectious particle has a multilayered architecture that is articulated upon an endoplasmic reticulum (ER)-derived inner envelope. This membrane acts as docking platform for the assembly of the outer icosahedral capsid and the underlying core shell, a bridging layer required for the formation of the central genome-containing nucleoid. While the details of outer capsid assembly are relatively well understood, those of core formation remain unclear. Here we report the functional characterization of pEP84R, a transmembrane polypeptide embedded in the inner envelope that surrounds the viral core. Using an ASFV recombinant inducibly expressing the EP84R gene, we show that absence of pEP84R results in the formation of non-infectious core-less icosahedral particles displaying a significant DNA-packaging defect. Concomitantly, aberrant core shell-like structures formed by co-assembly of viral polyproteins pp220 and pp62 are mistargeted to non-ER membranes, as also occurs when these are co-expressed in the absence of other viral proteins. Interestingly, co-expression of both polyproteins with pEP84R led to the formation of ER-targeted core shell-like assemblies and co-immunoprecipitation assays showed that pEP84R binds to the N-terminal region of pp220. Altogether, these results indicate that pEP84R plays a crucial role in core assembly by targeting the core shell polyproteins to the inner viral envelope, which enables subsequent genome packaging and nucleoid formation. These findings unveil a key regulatory mechanism for ASFV morphogenesis and identify a relevant novel target for the development of therapeutic tools against this re-emerging threat.

Functional Landscape of African Swine Fever Virus–Host and Virus–Virus Protein Interactions

“…While E120R is not essential for forming infectious intracellular virions, its inhibition drastically reduces the number of extracellular ASFV by impairing the transport of viral particles from the assembly sites to the plasma membrane [ 89 ]. The recently characterized inner envelope protein, EP84R, guides the formation of the core–shell by interacting with the polyprotein precursor CP2475L (pp220) [ 90 ]. Concomitantly, protein precursors CP2475L (pp220) and CP530R (pp62) are processed by the ASFV protease S273R, and their proteolytic products build the viral core–shell.…”

Asfarviruses and Closely Related Giant Viruses

“…Finally, the virus particle size is 260 nm, including the outer membrane. A recent study identified a transmembrane protein, pE84R, that plays a major role in the assembly of the inner core shell in interaction with polyproteins pp220 and pp62 [57]. ASFV is the only virus of this group with a described outer envelope produced by the host membrane and involved in the entry of the virus into cells (see Section 2.4).…”