Influenza type B neuraminidase can replace the function of type A neuraminidase

Influenza A and B viruses do not form reassortants with each other, presuma bly due to selection at either the RNA or protein level. Although differenc es in the promoter sequences of type A and B viruses have been studied, sel ection at the protein level has not been addressed. In this paper we descri be experiments to determine whether differences in structure and/or functio n of the neuraminidase (NA) protein preclude formation of A/B NA reassortan ts. influenza type A (N9) NA or B/Lee/40 NA expressed from plasmids can sup port multicycle growth of a NA-deficient type A virus (NWS-Mvi), indicating that their function in tissue culture is similar. To determine whether the type A or B NA supplied in trans can be incorporated into the Virion of NW S-Mvi, the virus grown in NA-expressing cells was purified by sucrose gradi ent centrifugation. In each case there was a peak of NA activity coincident with the virus peak, indicating that some NA protein is packaged into the virion. The experiments suggest that, in spite of large sequence difference s, the functions of the head, stalk, signal-anchor, and cytoplasmic domains of type A and B NAs are similar in tissue culture. Thus, lack of formation of A/B NA reassortant viruses is not due to restriction at the protein lev el. (C) 1999 Academic Press.