The CRM1-dependent NES motif in the matrix protein: another factor influencing Newcastle disease virus propagation and virulence.

As an important structural protein of Newcastle disease virus (NDV), the M protein plays a crucial role in the assembly and budding of the virus. In previous reports, we confirmed the existence of a classical nuclear export signal (NES) sequence on the M protein (i.e., amino acids 257 to 266), which plays an important role in the nuclear export of the protein. In this study, we found that the NES motif is associated with the evolutionary history of NDV, and found that the NES-mediated nuclear export efficiency of M proteins varies among different genotypes. Further research on the LaSota-NES and JSD0812-NES variants showed that the NES both affects the nuclear-cytoplasmic trafficking efficiency of M protein and influences the formation of virus-like particles (VLPs). The NES conforms to the chromosome region maintenance 1 (CRM1)-associated consensus leucine/isoleucine-rich NES motif, and our results confirm that the NES-mediated nuclear-cytoplasmic trafficking of M protein is dependent on the CRM1 pathway. At last, several recombinant LaSota strains were rescued using reverse genetics, with their NES sequence replaced by either a potent NES motif, or an R247K mutation, or both. The rescued rLaSota-QMS and rLaSota-FM strains showed elevated hemagglutination (HA) titers, increased 50% egg infective dose (EID) values, along with marginally higher mean death time (MDT) and intracerebral pathogenicity index (ICPI) values, all of which are attributable to their enhanced proliferation rates when compared to the wild-type (wt) rLaSota strain. Our findings contribute to a better understanding of the molecular mechanisms of replication and pathogenicity in NDV and, by extension, in other paramyxoviruses.