Nuclear import and DNA binding of human papillomavirus type 45 L1 capsid protein

During the life cycle of human papillomaviruses (HPVs), the L1 capsid prote ins seem to enter the nucleus twice: once after the virions infect the cell s, and later during the productive phase when they assemble the replicated HPV genomic DNA into infectious virions. We established for the high-risk H PV45 that when digitonin-permeabilized HeLa cells were incubated with L1 ho mopentameric capsomers, the HPV45 L1 protein was imported into the nucleus in a receptor-mediated manner. In contrast, intact capsids were not able to enter the nucleus. Immunoisolation assays showed that HPV45 L1 capsomers i nteract with cytosolic karyopherin alpha 2 beta 1 heterodimers. HPV45 L1 bo und strongly to karyopherin alpha 2, and weakly to karyopherin beta 1, as d id its nuclear localization signal (NLS). Nuclear import of HPV45 L1, or of a GST-NLSHPV45L1 fusion protein was efficiently mediated by karyopherin al pha 2 beta 1 heterodimers, and only weakly by karyopherin beta 1. Nuclear i mport required RanGDP, but was independent of GTP hydrolysis by Ran. Togeth er, these data suggest that the major nuclear import pathway for HPV45 L1 m ajor capsid protein in infected host cells is mediated by karyopherin alpha 2 beta 1 heterodimers and that GTP hydrolysis by Ran is not required for i mport. Remarkably, HPV45 L1 capsomers can interact nonspecifically with dif ferent types of HPV-DNA, and the DNA binding region of HPV45 L1 overlaps wi th its NLS sequence. (C) 2000 Wiley-Liss, Inc.