A ROLE FOR THE SENDAI VIRUS P-PROTEIN TRIMER IN RNA-SYNTHESIS

The SeV P protein is found as a homotrimer (P-3) when it is expressed in mammalian cells, and trimerization is mediated by a predicted coile d-coil motif which maps within amino acids (aa) 344 to 411 (the BoxA r egion). The bacterially expressed protein also appears to be trimeric, apparently precluding a role for phosphorylation in the association o f the P monomers. I have examined the role of P trimerization both in the protein's interaction with the nucleocapsid (N:RNA) template and i n the protein's function on the template during RNA synthesis. As with the results of earlier experiments (32). I found that both the BoxA a nd BoxC (aa 479 to 568) regions were required for stable binding of P to the N:RNA. Binding was also observed with P proteins containing les s than three BoxC regions, suggesting that trimerization may be requir ed to permit contacts between multiple BoxC regions and the N:RNA. How ever, these heterologous trimers failed to function in viral RNA synth esis, indicating that the third C-terminal leg of the trimer plays an essential role in P function on the template. We speculate that this f unction may involve the movement of P (and possibly the polymerase com plex) on the template and the maintenance of processivity.