The deduced amino acid (aa) sequence of L2 of the newly sequenced HPV51 strain, isolated by Matsukura and Sugase (Ma-strain), was markedly different from that of the prototype HPV51 isolated by Nuovo et al. (Nu-strain) (GenBank M62877) in two regions: aa 95-99 (region I) and aa 179-186 (region II). The two regions of Ma-strain were homologous to those of the other mucosal HPVs. The aa sequences of the N-terminal and C-terminal regions of Ma-L2 and Nu-L2 were identical and contained the nuclear localizing signal (NLS). When expressed in HEK293 cells, Ma-strain L2 (Ma-L2) was located in the nucleus but Nu-strain L2 (Nu-L2), in the cytoplasm. The chimeric L2s having both Nu-L2 regions I and II were located in the cytoplasm, and those having one of them were located both in the nucleus and cytoplasm, suggesting that Nu-L2 regions I and II inhibit the NLS function. For a better understanding of a role of L2 in infection, pseudovirion (PV) preparations were produced with a reporter, Ma-strain L1, and various L2s (Ma-L2, Nu-L2, or the chimeric L2s). These PV preparations contained structurally similar particles composed of L1 and L2 and the packaged reporter plasmid at a similar level. The reporter expression was not induced in HEK293 cells after inoculation with PVs containing the L2s that are incapable of localizing in the nucleus when expressed alone. Among PVs containing L2s capable of localizing in the nucleus, the reporter expression was induced only by PVs containing Ma-L2 region I. Thus, the results indicate that the expression of the reporter in the HPV51 PV requires the nuclear localizing ability of L2 and another unknown function associated with region I.