The 100K-Chaperone Protein from Adenovirus Serotype 2 (Subgroup C) Assists in Trimerization and Nuclear Localization of Hexons from Subgroups C and B Adenoviruses

Recombinant hexons from subgroup C adenoviruses (Ad2 and Ad5) and from a member of subgroup B (Ad3) adenoviruses have been expressed in insect cells. When expressed alone, all three hexons were found to be insoluble and accumulated as inclusion bodies in the cytoplasm. However, co-expression of recombinant Ad2, Ad5 or Ad3 hexon with Ad2 L4-100K protein resulted in the formation of soluble trimeric hexons. EM analysis of hexons revealed that they were indistinguishable from native hexon capsomers isolated from Ad2-infected human cells, or released from partially disrupted adenovirions. This suggests that 100K acts as a chaperone for hexon folding and self-assembly into capsomer in insect cells. Since 100K protein assists in the trimerization of subgroup C hexon, and of subgroup B hexon protein, it implies that it functions in a manner that is both homo- and heterotypic. During the course of recombinant protein expression, the 100K protein was found in association with hexon monomers and trimers within the cytoplasm. In the nucleus, however, 100K was found in complexes with hexon trimers exclusively. EM observation of purified 100K protein samples showed a dumb-bell-shaped molecule compatible with a monomeric protein. EM analysis of hexon-100K protein complexes showed that interaction of hexon with the 100K protein occurred via one of the globular domains of the 100K protein molecule. Our data confirm the role of the 100K protein as a scaffold protein for hexon, and provide evidence suggesting its function in hexon nuclear import in insect cells.