The crystal structure of the vesicular stomatitis virus nucleoprotein (N) in complex with RNA reveals extensive and specific intermolecular interactions among the N molecules in the 10-member oligomer. What roles these interactions play in encapsidating RNA was studied by mutagenesis of the N protein. Three N mutants intended for disruption of the intermolecular interactions were designed and coexpressed with the phosphoprotein (P) in an Escherichia coli system previously described (T. J. Green et al., J. Virol. 74:9515-9524, 2000). Mutants N (Delta1-22), N (Delta347-352), and N (320-324, (Ala)(5)) lost RNA encapsidation and oligomerization but still bound with P. Another mutant, N (Ser290-->Trp), was able to form a stable ring-like N oligomer and bind with the P protein but was no longer able to encapsidate RNA. The crystal structure of N (Ser290-->Trp) at 2.8 A resolution showed that this mutant can maintain all the same intermolecular interactions as the wild-type N except for a slight unwinding of the N-terminal lobe. These results suggest that the intermolecular contacts among the N molecules are required for encapsidation of the viral RNA.