Human papillomavirus 16 minor capsid protein L2 helps capsomeres assemble independently of intercapsomeric disulfide bonding

The human papillomavirus (HPV) capsomeres (pentamers of major capsid protein L1), which constitute along with L2 the virion capsid, can assemble themselves alone into the L1-capsid particles in vivo and in vitro, depending on intercapsomeric disulfide bonds. To study a possible role of L2 in capsid assembly, we examined the interaction between HPV16 L2 and capsomeres under the conditions that inhibit the formation of disulfide bonds in vitro and in vivo. The purified L2 bound to free capsomeres prepared by disassembling L1-capsids but not to the L1-capsids in vitro. And the L2 was found to help capsomeres assemble into smaller capsid-like particles independently of intercapsomeric disulfide bonding. Similar particles were obtained from the Sf9 cells co-infected with baculoviruses expressing L2 and an L1 mutant that lacks a C-terminal cysteine (C428S) and can form capsomeres but no capsids when expressed alone. These findings suggest that L2, which is known to bind both viral DNA and L1, may contribute to the formation of the virion by linking viral DNA and capsomeres and by helping capsomeres assemble before the virion capsid structure is completed by dintercapsomeric disulfide bonding.