Foot-and-mouth disease virus VP4: roles in membrane permeability and capsid breathing

Foot-and-mouth disease virus (FMDV) is an economically-important picornavirus that infects cloven hooved animals. Entry is a vital step in the virus lifecycle and results in conformational changes in the capsid, including release of the capsid protein VP4. For other picornaviruses, VP4 has been shown to induce membrane permeability and is hypothesised to be involved in the transfer of the viral genome into the cytoplasm using a mechanism which remains poorly understood. The studies presented in this thesis aimed to explore the role of FMDV VP4.
Firstly, characterisation of the membrane permeability induced by FMDV VP4 demonstrated, in contrast to previous work with human rhinovirus (HRV) VP4, that both termini of FMDV VP4 independently induced size-selective membrane permeability. Secondly, the hepatitis B core virus-like particle (VLP) was used to display the N-terminal 15 amino acids of FMDV VP4. Two approaches to displaying this sequence on the VLP were successfully established and taken forward to an immunisation study. Thirdly, characterisation of VP4 reactive antibodies has further alluded to the role of FMDV VP4. Alongside the antibodies generated against the VLPs, antibodies against the N- and C-terminus of VP4 were generated using KLH-peptide conjugates. In contrast to previous studies with antibodies raised against HRV VP4, the FMDV VP4 antibodies did not affect FMDV infectivity or affect FMDV-induced membrane permeability. However, the VP4 antibodies detected FMDV by ELISA, indicating externalisation of VP4 from the capsid. The process of transient VP4 externalisation, termed capsid breathing, has been demonstrated for other picornaviruses and this work indicated capsid breathing occurs in FMDV.
The work presented here indicates that FMDV VP4 plays a role in membrane permeability and capsid breathing. Furthermore, these processes may be shared between different genera of picornaviruses.