Sequences required for the nuclear targeting and accumulation of human papillomavirus type 6B L2 protein

The L2 protein of the human papillomaviruses is a minor structural protein and is not necessary for the major L1 protein to assemble into capsids. However L2 protein binds DNA and enhances the efficiency of HPV capsid assembly, suggesting an important role in the production of infectious papillomaviruses. L2 accumulates rapidly in the nucleus after synthesis in the cytoplasm. To identify L2 sequences responsible for nuclear targeting and accumulation, full-length HPV6bL2 protein and L2 proteins containing sequence deletions were expressed in CV-1 cells, using recombinant vaccinia viruses. beta gal protein fused to C-terminal and N-terminal L2 sequences were localized in the nucleus and demonstrated that both the C-terminal putative nuclear localization signal (NLS) and N-terminal DNA binding sequences of the L2 protein, which are rich in arginine and lysine, are functional in targeting proteins into the nucleus. L2 mutants lacking amino acids (aa 286-306) do not accumulate in nucleus even when the C-terminal NLS and N-terminal DNA binding sequences are intact. Accumulation in the nucleus of L2 protein lacking aa 286-306 could be also achieved by increasing the L2 protein size via insertion of a portion of L1 protein. Amino acid sequence alignment of L2 proteins from sequenced papillomaviruses showed that sequences in this region are relatively conserved. Our results indicate that in the L2 protein, the nuclear targeting and accumulation are controlled by two different sequences and the rapid nuclear translocation and accumulation may play an important role in papillomavirus life cycle.