The influenza A virus matrix 1 protein (M1) shuttles between the cytoplasm and the nucleus during the viral life cycle and plays an important role in the replication, assembly, and budding of viruses. Here, a leucine-rich nuclear export signal (NES) was identified specifically for the nuclear export of the M1 protein. The predicted NES, designated the Flu-A-M1 NES, is highly conserved among all sequences from the influenza A virus subtype, but no similar NES motifs are found in the M1 sequences of influenza B or C viruses. The biological function of the Flu-A-M1 NES was demonstrated by its ability to translocate an enhanced green fluorescent protein (EGFP)-NES fusion protein from the nucleus to the cytoplasm in transfected cells, compared to the even nuclear and cytoplasmic distribution of EGFP. The translocation of EGFP-NES from the nucleus to the cytoplasm was not inhibited by leptomycin B. NES mutations in M1 caused a nuclear retention of the protein and an increased nuclear accumulation of NEP during transfection. Indeed, as shown by rescued recombinant viruses, the mutation of the NES impaired the nuclear export of M1 and significantly reduced the virus titer compared to titers of wild-type viruses. The NES-defective M1 protein was retained in the nucleus during infection, accompanied by a lowered efficiency of the nuclear export of viral RNPs (vRNPs). In conclusion, M1 nuclear export was specifically dependent on the Flu-A-M1 NES and critical for influenza A virus replication.
Influenza A virus is a negative-strand RNA virus composed of eight segmented strands of an RNA genome (21). The viral structural components include the viral envelope, the transmembrane proteins (hemagglutinin [HA], neuraminidase [NA], and M2), M1, NS2, and the viral ribonucleoprotein (vRNP), which contains viral RNA (vRNA), nucleoprotein (NP), and viral polymerase proteins (PB1, PB2, and PA) (44).M1 is the most abundant protein in the virus particle, sustaining the virion structure by forming a shell connecting the viral envelope and the nucleocapsid (40). M1 is synthesized in the late stages of infection, shuttles between the nucleus and the cytoplasm (45), and plays multiple roles in various steps of the influenza virus life cycle.Newly synthesized M1 is transported from the cytoplasm into the nucleus via a nuclear localization signal (NLS) located in its N-terminal domain (residues 101 to 105) (38, 48). In the nucleus, M1 is involved in the blocking of the transcription of viral mRNA by binding to vRNPs (4, 49). Aside from terminating viral transcription, M1 also plays an important role in the nuclear export of vRNPs. Indeed, the nuclear presence of M1 is required for vRNPs to be transported to the cytoplasm (8, 27), where the vRNPs are subsequently transported to the budding site. First, M1 binds to histones in the nucleus (51) and may participate in releasing vRNPs from the nuclear matrix. Second, M1 likely bridges the NEP and vRNPs, forming a complex that is in turn exported from the nucleus by the nuclear export signal (NES) ...