Nuclear localization of turnip crinkle virus movement protein p8

Turnip crinkle virus (TCV) is a single-stranded positive-sense RNA virus of the Carmovirus genus. Two of its five open reading frames (ORFs), encoding proteins of 8 and 9 kDa, are required for cell-to-cell movement of the vir us. These movement proteins (MPs) were fused to green fluorescent protein ( GFP) to determine their cellular localization. In protoplasts, p9-GFP, like GFP itself, is found throughout the cytoplasm, as well as in cell nuclei. In contrast, p8-GFP was confined to the cell nucleus. Similar localization patterns were observed when specific small peptide epitopes were fused to p 8 and p9 proteins instead of GFP. The cytoplasmic localization of pS-GFP an d nuclear localization of p8-GFP were also detected in leaves after particl e bombardment of DNA encoding these fusion proteins or after overexpression of p8-GFP In transgenic Arabidopsis seedlings, The expression of the GFP f usion proteins by recombinant TCV viruses in Infected protoplasts or on ino culated Arabidopsis leaves produced similar patterns. Unlike TMV-NIP and ot her MPs studied to date, no obvious punctuate expression in the cell wall o r association with the cytoskeleton was detected. The sequence analysis of p8 revealed two unique nuclear localization signals (NLSs), which were not conserved within p8 homologues of other viruses in the genus Carmovirus. Mu tation in either of these NLSs did not disrupt the nuclear localization of p8-GFP. However, when both NLSs were mutated, p8-GFP expression was no long er restricted to cell nuclei. The NLSs are not required for cell-to-cell mo vement; TCV recombinant viruses mutated in one or both NLSs could still fac ilitate cell-to-cell movement of the virus. The nuclear localization of p8 suggests a novel function for this protein in the cell nucleus, (C) 2000 Ac ademic Press.