156 Samji: Influenza A: understanding the viral life cycle viral RNA, leading to steric blockage at the end of viral RNA synthesis [36,37]. Each viral segment has a stretch of five to seven U residues approximately 17 nucleotides from the 5’ end, and this forms the basis of the viral polyadenylation signal [38]. Therefore, polyadenylation of the viral mRNAs occurs due to a stuttering mechanism, whereby the RdRp moves back and forth over this stretch of U residues, leading to the formation of a poly(A) tail [39,40]. Interestingly, NS1 in- hibits the nuclear export of cellular mRNAs by preventing cellular mRNAs from being cleaved at the polyadenylation cleavage site [41]. It does this by binding to the CPSF [42] and poly(A) binding protein II (PABPII), which is involved in stimulating poly(A) polymerase to add the poly(A) tail onto newly cleaved mRNAs [43]. EXPORT OF vRNPs FROM THE NUCLEUS It is known that only negative sense vRNPs are exported from the nucleus [44]. vRNPs appear to be exported out of the nu- cleus via the CRM1 dependent pathway through the nuclear pores. NP has been shown to interact with CRM1 directly, although no GTP hydrolysis activity could be detected. This indicates an unusual method of export if the binding of NP to CRM1 is critical for ex- port of the vRNPs. M1 is known to interact directly with the vRNPs through the C-termi- nal end of the protein. Interestingly, the N-ter- minal portion of the protein is known to have an NLS potentially involved in the import of the vRNPs. It has been shown that the N-ter- minal portion of M1 can bind to NEP, thus masking the NLS. NEP also has been shown to bind to CRM1 with the accompanying GTP hydrolysis that normally occurs in a CRM1- dependent export pathway. Therefore, it is hy- pothesised that M1 binds to the negative sense vRNPs, as well as binding to NEP. In turn, NEP binds to CRM1, and through this “daisy- chain” complex, the vRNPs are exported out of the nucleus [12,45,46]. Recently, live imaging has been em- ployed to visualize the movement of vRNPs during the influenza life cycle. It has been shown that NP preferentially localizes to the apical side of infected nuclei, indicating po- larized exit of the viral genome [47,48]. ASSEMBLY AND BUDDING AT THE HOST CELL’S PLASMA MEMBRANE After the vRNPs have left the nucleus, all that is left for the virus to do is form viral particles and leave the cell. Since influenza is an enveloped virus, it uses the host cell’s plasma membrane to form the viral particles that leave the cell and go on to infect neigh- bouring cells. It is possible to create virus particles that do not contain any or only a few vRNPs, but all the viral proteins nor- mally found within the viral lipid bilayer, i.e., HA, NA, and M2, must be present to form a viral particle [4]. Virus particles bud from the apical side of polarized cells [4]. Because of this, HA, NA, and M2 are transported to the apical plasma membrane. It has been shown through deletion and mutational analysis that the tail of M2 is extremely important in the formation of viral particles. Viruses that had the M2 tail deleted or partially mutated produced elongated particles [49]. M1, which is present underneath the lipid bilayer, is important in the final step of closing and budding off of the viral particle [6,50]. Sev- eral host factors are involved in the budding off of viruses from plasma membranes, and these are reviewed in [4,51]. There are two models that have been hy- pothesized to explain the packaging of viral genomic segments into virions: the random packaging model [52,53] and the specific packaging model [54]. The former predicts that viral genomic segments are randomly packaged into virions; whereas, the latter predicts that there are signals present in the viral segments dictating which segments are to be packaged into the virions. Packaging signals have been identified in the 5’ and 3’ non-coding and coding regions of some of the viral segments [55-59], thus leaning to- ward the specific packaging model. One of the most important steps that must occur before the newly made viral par- ticle can leave the plasma membrane is the cleavage of sialic acid residue from glyco-