Table of Contents:
The arenavirus nucleoprotein (NP) plays a central role during viral replication. It encapsidates the bi-segmented viral RNA-genome and is involved in regulating the viral transcription and replication. Furthermore, it interacts with the viral matrix protein and is responsible for the inhibition of type I interferon response during viral infection. Despite its importance for the replication of arenaviruses, no studies were available concerning either the homo-oligomerization of the nucleoprotein and its influence on the different functions of the nucleoprotein or the identification of functional domains within the nucleoprotein. In the present study, it could be shown that the arenavirus nucleoprotein forms stable, ring-like trimers in the absence of viral RNA. The formation of these trimers is independent of post-translational modifications and conserved within the family arenaviridae. Amino acids in the N- and C-terminal domains have been identified to be critical for nucleoprotein trimerization. In addition, homo-oligomerization of the nucleoprotein is required for its function during replication and transcription of the viral genome. In contrast, NP self-interaction does not play any role for its interaction with the viral matrixprotein or inhibition of the type I interferon response. Besides this, the nucleoprotein is proteolytically truncated by cellular proteases. The finding that large fragments resulting from this cleavage are no longer able to oligomerize suggests that they might function as inhibitors of the type I interferon response during viral replication, while uncleaved nucleoprotein is used for replication, transcription and encapsidation of the viral genome. The nucleoprotein facilitates incorporation of the viral genome into budding virions through its binding to the viral matrix protein. In the present study, the C-terminal domain of the nucleoprotein were identified to be crucial for this interaction. Based on these observations, a model for the association between NP and the matrix protein at viral membranes is proposed. The analysis of nucleoproteins and matrix proteins from different arenaviruses showed that heterotypic interactions are only possible when both proteins originate from closely related arenaviruses. Thus, the interaction between the nucleoprotein and the matrix protein has been identified as a factor limiting the formation of reassortants between different arenaviruses. In summary, the present work shows the relationship between homo-oligomerization and the four known biological functions of the arenavirus nucleoprotein. Furthermore, functional domains responsible for homotypic nucleoprotein-nucleoprotein interactions and the interaction between the nucleoprotein and the matrix protein were identified.