Structural insights into reptarenavirus cap-snatching machinery

Cap-snatching was first discovered in influenza virus. structures of the involved domains of the influenza virus polymerase,namely the endonuclease in the pa subunit and the cap-binding domain in the pb2 subunit,have been solved. cap-snatching endonucleases have also been demonstrated at the very n-terminus of the l proteins of mammarena-,orthobunya-,and hantaviruses. however,a cap-binding domain has not been identified in an arena- or bunyavirus l protein so far. we solved the structure of the 326 c-terminal residues of the l protein of california academy of sciences virus (casv),a reptarenavirus,by x-ray crystallography. the individual domains of this 37-kda fragment (l-cterm) as well as the domain arrangement are structurally similar to the cap-binding and adjacent domains of influenza virus polymerase pb2 subunit,despite the absence of sequence homology,suggesting a common evolutionary origin. this enabled identification of a region in casv l-cterm with similarity to a cap-binding site; however,the typical sandwich of two aromatic residues was missing. consistent with this,cap-binding to casv l-cterm could not be detected biochemically. in addition,we solved the crystal structure of the corresponding endonuclease in the n-terminus of casv l protein. it shows a typical endonuclease fold with an active site configuration that is essentially identical to that of known mammarenavirus endonuclease structures. in conclusion,we provide evidence for a presumably functional cap-snatching endonuclease in the n-terminus and a degenerate cap-binding domain in the c-terminus of a reptarenavirus l protein. implications of these findings for the cap-snatching mechanism in arenaviruses are discussed. © 2017 rosenthal et al.