Besides the central polymerase domain, the viral L protein contains an endoribonuclease in its N-terminus. Hantaviruses, which are another group of important pathogens within the order of Bunyavirales, contain an endonuclease, which is too active to be recombinantly produced: it degrades all RNA making it toxic for the expressing cell. We studied the endonuclease in detail by introducing a set of mutations, which result in an attenuated enyzme, allowing for expression in E. coli. By determination of the crystal structure we could propose a role of all mutated amino acids and gain new insights into an enzyme, which is essential for the viral life cycle and therefore an attractive drug target.
Furthermore, we solved the structures of the N- and C-terminal domains of the California Academy of Sciences virus (reptarenavirus) L protein. The N-terminal domain of this L protein contains an endoribonuclease, which is structurally homologous to already published arenavirus endonucleases. The C terminus of the L protein contains a putative cap-binding site very similar to influenza virus cap-binding protein PB2. Both of these functions, cap-binding and endonuclease, are required for the cap-snatching mechanism, by which segmented negative strand RNA viruses presumably initiate transcription. In our structure however, the residues potentially involved in cap-binding did not show the expected conformation and also functional studies could not proof a cap-binding activity leaving it unclear how cap-snatching actually works in bunyaviruses.