Das Ubiquitin-Proteasom-System und die Ubiquitin-E3-Ligase Nedd4 sind involviert in den Abbau der RNA-Polymerase II durch den Virulenzfaktor NSs des La Crosse-Virus

The La Crosse virus (LACV) belongs to the family Bunyaviridae, one of the biggest virus families, and is grouped together with the prototype Bunyamwera virus (BUNV) and for example the Schmallenberg virus (SBV) into the genus Orthobunyavirus. The virus is transmitted by different mosquito species and is a major health problem in the mid-western and eastern states of the US due to the prevalence of infected mosquitoes in these areas. The virus can cause severe encephalitis and meningitis, especially in children under the age of 15 years. Such an encephalitis and meningitis can result in deleterious consequences including learning disabilities, cognitive deficits, hyperactivity, seizures, and epilepsy. LACV belongs to the negative-strand RNA viruses with a tri-segmented genome and encodes for a virulence factor, the NSs (non-structural protein encoded on the S segment) protein, which inhibits the antiviral interferon (IFN) induction of the infected host. It does this by selectively triggering the proteasomal degradation of the Rpb1 subunit of RNA polymerase II (RNAPII). This degradation shows similarities to the DNA damage response (DDR), during which Rpb1 is ubiquitinylated by sequentially acting ubiquitin E3 ligases and is consequentially targeted by the proteasome. Here, the ubiquitin E3 ligase Nedd4 (neural precursor cell expressed developmentally down-regulated protein 4) plays a major role. The following work demonstrates that the LACV-NSs protein induces a general ubiquitinylation and that this ubiquitinylation is lysine48 (K48) linked. Furthermore, it could be shown that specifically the Rpb1 subunit of RNAPII is ubiquitinylated. Moreover, it was demonstrated that the ubiquitin E3 ligase Nedd4 is involved in the degradation of Rpb1, as an siRNA-induced knockdown of Nedd4 partially rescues the degradation of Rpb1 and thus the inhibition of the IFN induction. In addition, it was shown that the subcellular localization of Nedd4 is changed by the expression of the LACV-NSs protein, from a normally homogenous distribution throughout the cell to a perinuclear localization. Finally, a transcriptomice analysis of the general host response to LACV was performed. Surprisingly, it became evident that the LACV-NSs protein, despite being a global suppressor of RNAPII, specifically targets genes of the innate immune system.