Bluetongue virus produces large numbers of tubules during infection. The tubules are formed from a 552-amino-acid, 64-kDa NS1 protein encoded by the viral double-stranded RNA segment M6. A series of deletion and extension mutants of bluetongue virus serotype 10 NS1 has been generated and expressed in insect cells in order to identify the carboxy-terminal components of the protein which are important for tubule formation. The mutants AcCT5 and AcCT10, lacking 5 and 10 of the carboxy-terminal

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... dues, respectively, were prepared. By analyzing their abilities to form tubules, it was shown that AcCT5 was capable of this function whereas AcCT10 was not, indicating that the last five amino acids are not strongly involved in NS1 tubule formation. Extension mutants including foreign antigenic sequences involving up to 16 amino acids added to the C terminus of NS1 were shown to form tubules, although an extension of 19 amino acids inhibited tubule formation. Analysis of a panel of monoclonal antibodies has established that an NS1 antigenic site is located near the carboxy terminus of the protein. It appears to be exposed on the surface of tubules. The opportunities to develop new vaccines using recombinant NS1 to deliver foreign epitopes are discussed.