ABSTRACT

The icosahedral capsid of hepatitis B virus (HBV), serologically dened as hepatitis B core antigen (HBcAg), has become one of the most attractive viral nanoparticles for surface display of heterologous molecules, largely due to its exceptional immunogenicity and the easy accessibility of recombinant wild-type (wt) and modied versions of the constituent core protein (HBc) in the form of capsid-like particles (CLPs); we will use the term CLP rather than virus-like particle (VLP) in this article because in HBV, the envelope around the capsid denes the appearance of virions. While various aspects of HBc as a particulate carrier are covered in separate chapters of this book (Chapters 2, 11, and 13), the focus here is on a newly developed HBc-based display system termed SplitCore. Its fundamentally distinct feature is the ability of two separately expressed fragments of the HBc to efciently associate into a 3D structure that retains the assembly competence of the wt protein; however, the interruption in the central part of the primary sequence forming the most surface-exposed region of the particle structure provides for an unprecedented exibility in the choice of molecules that can be presented [1]. In the following, we outline the various

rationales that led to the SplitCore concept and provide examples for SplitCore CLPs presenting foreign proteins that are incompatible with the structural constraints of the conventional contiguous chain HBc system. We also discuss options to engineer multilayered particles and the potential advantages of the open-ended versus clamped-in display of smaller peptides. Finally, we examine current limitations of the SplitCore system and how they may be overcome. We hope this will also spur the interest of those working on other proteinaceous nanoparticles to explore the extended capabilities the split protein approach can offer.