Complex formation with vesicular stomatitis virus phosphoprotein NS prevents binding of nucleocapsid protein N to nonspecific RNA.

The interactions between the nucleocapsid protein N and either RNA or the phosphoprotein NS of vesicular stomatitis virus (VSV) were studied by the transcription of N and NS mRNAs from SP6 vectors, followed by translation in a rabbit reticulocyte lysate. Nascent N protein bound tightly to added labeled RNA, as well as to endogenous RNA in the reticulocyte lysate. This binding was demonstrated by three independent techniques. First, labeled N protein and labeled RNA migrated identically as a series of sharp, closely spaced bands in a nondenaturing gel system. Second, translated N protein behaved as a stable ribonucleoprotein complex in CsCl gradients and sedimented to the same density as the authentic N-RNA template of VSV. Third, translated N protein protected a series of labeled RNA fragments from digestion by RNase A. None of the three RNA-binding criteria was satisfied by either translated NS protein or two deletion mutants of N protein or by other components of the reticulocyte lysate. The evidence suggests that the observed binding of RNA by nascent N was not RNA sequence specific, in contrast to the encapsidation process during VSV replication. Moreover, the prior formation of N-NS complexes totally abolished the observed binding of RNA by N. Thus, we propose that NS may be responsible for conferring the sequence specificity of the RNA binding that occurs during VSV genome replication.