ELEVATED EXPRESSION OF THE HUMAN PARAINFLUENZA VIRUS TYPE-1 F-GENE DOWN-REGULATES HN-EXPRESSION

Interactions involved in the expression of parainfluenza glycoproteins were examined by expressing cDNA clones of the HN and F genes from hu man parainfluenza virus type-1 (hPIV1) or Sendai virus (SV) in recombi nant Semliki Forest virus (recSFV) or vaccinia-T7 expression vectors. We found that expression of a cloned F protein gene of hPIV1 resulted in downregulation of the HN proteins of hPIV1 or SV. Compared to the a mount of HN expressed in the absence of F, coexpression of HN and F le d to about 70% reduction in HN. This reduction of HN was observed in b oth total cell lysates and in protein localized on the cell surface. I n contrast to hPIV1 F, SV F did not suppress the expression of HN. Nor thern blot analysis indicated that similar levels of HN mRNA accumulat ed in the absence or presence of hPIV1 F. The reduction of HN protein expression by hPIV1 F was detectable after as little as a in-min label ing period, suggesting that downregulation occurred at the level of tr anslation or at an early stage of protein folding. In hPIV1-infected c ells, the amount of F protein synthesized was only about 15% of that o f HN, whereas SV F is expressed at high levels. When the level of F in hPIV1-infected cells was artificially increased by recSFV, HN express ion was suppressed. The reduction of F protein production in hPIV1-inf ected cells was regulated at the level of transcription. Characterizat ion of mRNAs produced in hPIV1-infected cells showed that only 20% of the hPIV1 F mRNAs were monocistronic transcripts; 80% were bicistronic M-F readthrough mRNAs. Because proteins are suggested to be synthesiz ed from only the first cistron of bicistronic mRNA in paramyxovirus (T . C. Wong and A. Hirano (1987) J. Virol. 61, 584-589), production of F protein is likely suppressed by transcriptional regulation in hPIV1-i nfected cells. These results suggest that F is capable of downregulati ng the synthesis of HN, but that this is normally prevented in hPIV1-i nfected cells by suppression of F protein synthesis by transcriptional regulation. (C) 1997 Academic Press.