Identification of the nuclear localization signal of human papillomavirus type 16 L1 protein

Human papillomavirus type 16(HPV16) L1 and L2 capsid proteins can be detected only in the nucleus of infected cells. For other nuclear proteins, specific sequences of basic amino acids(aa) termed nuclear localization signals (NLS) direct the protein from the cytoplasm to the nucleus. We used a series of deletion and substitution mutations of the HPV16 L1 protein, produced by recombinant vaccinia virus (rVV), to identify NLS within HPV16 L1 and showed that HPV16 L1 contains two NLS sequences, each containing basic aa clusters. One NLS consisted of 6 basic amino acids (KRKKRK from aa 525 to 530) at the carboxy terminal end of L1. The other NLS contained 2 basic aa clusters(KRK from aa 510 to 512 and KR at aa 525, 526) separated by 12 amino acids. Mutations in either NLS did not alter nuclear localization of L1 when the other remained intact, but mutations to both prevented nuclear localization of L1. The L1 NLS could be overridden by introduction of a membrane binding sequence at the amino terminal end of the protein. A databases search showed that all sequenced papillomaviruses are predicted to have L1 and L2 capsid proteins with sequences of basic amino acids homologous with one or both NLS of HPV16 L1.