Papillomaviruses are non-enveloped DNA tumor viruses. Infection by papillomaviruses is associated with tumorigenesis in experimental animals and in humans (reviewed in ). The virus particle is comprised of 72 pentamers of the major capsid protein (L1) on the outer surface, arranged on a T = 7 icosahedral lattice (reviewed in [3,5]). A minor capsid, L2, is located internal to the L1 shell. The viral genomic DNA is packaged within the L1/L2 capsid as a minichromosome (reviewed in ).
The recombinant L1 capsid alone can assemble into the virus-like particle structure in vivo and in vitro [4,6,7,10-12,15]. Thus, the L1 protein contains all the information needed for the particle assembly. The in vitro assembly of L1 occurs spontaneously under high salt or low pH conditions [1,2]. The sizes of particle assembly can be regulated by N-terminal truncations. For example, HPV16 L1 lacking the first 10 residues assemble into T = 1 particle, but L1 deletions missing nine or fewer residues from the N-terminus assemble into T = 7 particles [1,2].
The atomic structure of a T = 7 papilloma virion has not yet been determined. However, the crystal structure of a smaller particle structure, a T = 1 particle containing 12 pentamers, has been solved . The high resolution T = 1 structure reveals the L1 pentamer structure and the pentamer-pentamer interactions essential for the assembly of the T = 1 particle. The L1 pentamer has five elbow-like lateral projections that are composed of the approximately 100 residues at the C-terminus (Fig. 1A). Each projected elbow consists of an α-helix (helix 4 or h4) anchored to the core structure through two other helices, h2 and h3 (Fig. 1B). Pentamers are linked through strong hydrophobic interactions between h4 on one pentamer with h2 and h3 of a neighbor (Fig. 1C). The remaining C-terminal residues return to the L1 core to form a fifth helix, h5 (Fig. 1B).
By fitting the L1 atomic structure of from the T = 1 particle into the cryo-EM reconstruction of BPV , an atomic model for T = 7 assembly was proposed . In the fitted T = 7 model, the laterally projected C-terminal fold of L1 seen in the T = 1 structure is rearranged to extend into the neighboring pentamers. This C-terminal arm exchange between the neighboring pentamers is similar to the "invading arm" model of the polyomavirus VP1 T = 7 virion structure [8,13]. However, the roles of the helix-helix interactions observed in the T = 1 structure are not defined in the T = 7 assembly model. In order to understand the functional roles of h2, h3, and h4 in T = 7 assembly, we have made structure-guided internal deletions of these helices on the L1 of two different HPV types. We have demonstrated that h2 and h3 are essential for L1 folding and the production of the pentameric L1 and that h4 is indispensable for the assembly of T = 7 particles.