Influenza virus, a negative strand RNA virus, cannibalizes host cell, capped RNA polymerase II transcripts in the nucleus via a process termed "cap-snatching". The viral transcriptase enzyme; which is composed of a complex of the three viral polymerase (P) proteins, contains a cap-dependent endonuclease that cleaves capped cellular RNAs in the nucleus 10-13 nucleotides from their 5' ends. The resulting capped RNA fragments are required as primers for the initiation of viral mRNA synthesis. In the 18 year since the discovery of "cap-snatching" it has not been determined how the viral transcriptase exhibits selectivity and "snatches" caps from cellular, but not viral, mRNAs. Here we elucidate the surprising mechanism of this selectivity: the complex of the same three viral P proteins that catalyzes "cap-snatching" is also responsible for selectivity protecting the 5' ends of viral, but not cellular, mRNAs from "cap-snatching". The viral P protein complex is able to acquire these two very different functions because this complex lacks any detectable activity unless it binds to one or more specific RNA sequences. Here we demonstrate that the viral P protein complex binds to the common sequence in all the viral mRNAs that is immediately 3' to the 5' sequence that is "snatched" from host cell RNAs. This binding activates the cap-binding activity of the P protein complex, thereby enhancing its binding to the capped viral mRNA. We show that these P protein complexes protect the 5' ends of viral mRNAs from endonucleolytic cleavage by the viral transcriptase, whereas the 5' ends of nonviral mRNAs are not protected.
