The 1.51-Å structure of the poxvirus L1 protein, a target of potent neutralizing antibodies

Abstract

Although eradicated from nature more than two decades ago, the threat of smallpox has reemerged because of concerns over its use as a biological weapon. We present the structure of the poxvirus L1 protein, a molecule that is conserved throughout the poxvirus family and is nearly identical in vaccinia virus and in variola virus, which causes smallpox. L1 is a myristoylated envelope protein that is a potent target for neutralizing antibodies and an important component of current experimental vaccines. The L1 structure reveals a hydrophobic cavity located adjacent to its N terminus. The cavity would be capable of shielding the myristate moiety, which is essential for virion assembly. The structure of L1 is a step in the elucidation of molecular mechanisms common to all poxviruses that may stimulate the design of safer vaccines and new antipoxvirus drugs.myristoylation ͉ virion protein ͉ x-ray structure P oxviruses are a family of large DNA viruses, the most infamous of which is variola virus, the cause of smallpox. After the worldwide eradication of smallpox more than two decades ago by immunization with the closely related vaccinia virus, routine immunization against smallpox ceased (1). With the threat of an intentional release of variola virus and the emergence of monkeypox virus that also infects humans, the development of antiviral drugs and safer vaccines has been urged (2). By better understanding mechanisms involved in poxvirus biology, therapeutics and vaccines can be designed to act against the whole family of poxviruses (3).Poxviruses have Ϸ200 genes and a complex life cycle. After entry into the host cell, the virus reproduces in the cytoplasm, encoding its own replication machinery and transcription factors. Maturing vaccinia virions undergo a transformation from spherical immature particles to the brick-shaped intracellular mature virions (IMVs) that have lipid membranes and are the first infectious form of the virus. Most IMVs are released directly by lysis of the host cell. Some IMVs acquire additional membranes and are then transported out through the host cell membrane. They can either remain attached to the outside of the cell or detach from the cell as an extracellular enveloped virus (EEV). Disruption of the EEV exterior membrane, either by mechanical force or by host complement factors, can also release the IMV form (4). Vaccine development is targeting proteins from each form (5, 6), although because of the fragile nature of the EEV outermost membrane, the IMV form of the virus is thought to play the major role in host-to-host transmission (7).L1 (also called L1R) is a myristoylated transmembrane protein of 250 residues that is expressed on the surface of the IMV form of the virus. It is an essential protein because vaccinia viruses with the L1 gene deleted are not capable of maturation (8). Although L1 has a C-terminal hydrophobic segment embedded in the viral membrane, the 185-residue, disulfide-bonded ectodomain is located in the cytoplasm before lysis of the host cell. In the otherwise re...

Targeting the vaccinia virus L1 protein to the cell surface enhances production of neutralizing antibodies

“…L1 contains three disulfide bonds normally formed by a virus-encoded disulfide bond formation pathway [32,33]. The requirement for disulfide bonds in the interaction of several potently neutralizing monoclonal antibodies, including MAb-7D11 and MAb-10F5, has been reported [30,34].…”

“…Antibodies against L1 can neutralize viral infectivity, suggesting that L1 may also play a role in particle entry either directly or indirectly [30]. The structure of L1 has been solved and reveals a molecule comprised of a bundle of ␣-helices packed against a pair of two-stranded ␤-sheets, held together by four loops [32]. The structure also contains three disulfide bonds that are formed in the cytoplasm by a virus-encoded disulfide bond formation pathway [33].…”

Vaccinia Virus F9 Virion Membrane Protein Is Required for Entry but Not Virus Assembly, in Contrast to the Related L1 Protein

“…Both L1 and F9 are conserved in all poxviruses, suggesting that they have nonredundant functions. The atomic structure of the L1 protein has been determined (35), whereas the F9 protein has not been characterized except to demonstrate that it has intramolecular disulfide bonds formed by the poxvirus-encoded redox system (34). Here, we show that the F9 protein is located at the surface of the MV membrane and that F9, unlike L1, is not required for morphogenesis but has an essential role in virus entry and cell-cell fusion.…”

Disparity between Levels of In Vitro Neutralization of Vaccinia Virus by Antibody to the A27 Protein and Protection of Mice against Intranasal Challenge

“…L1 is a myristylated transmembrane component of the MV (24,69). Considerable information is available regarding its structure and one of its neutralization epitopes (60,61). Although L1 was reported to be essential for virus assembly (55), recent data demonstrate that the primary role of L1 is in virus entry (H. Bisht and B. Moss, unpublished data).…”