Mapping and Engineering Functional Domains of the Assembly-Activating Protein of Adeno-associated Viruses.

Adeno-associated viruses (AAVs) encode a unique assembly-activating protein (AAP) within their genomes that is essential for capsid assembly. Studies to date have focused on establishing the role of AAP as a chaperone that mediates the stability, nucleolar transport, and assembly of AAV capsid proteins. Here, we map structure-function correlates of AAP using secondary structure analysis, followed by deletion and substitutional mutagenesis of specific domains, namely, the N-terminal hydrophobic region (HR), conserved core (CC), proline-rich region (PRR), threonine/serine-rich region (T/S), and basic region (BR). First, we establish that the centrally located PRR and T/S are flexible linker domains that can either be deleted completely or replaced by heterologous functional domains that enable ancillary functions such as fluorescent imaging or increased AAP stability. We also demonstrate that the C-terminal BR domains can be substituted with heterologous nuclear or nucleolar localization sequences that display various abilities to support AAV capsid assembly. Further, by replacing the BR domain with immunoglobulin (IgG) Fc domains, we assessed AAP complexation with AAV capsid subunits and demonstrate that the hydrophobic region (HR) and the conserved core (CC) in the AAP N terminus are the sole determinants for viral protein (VP) recognition. However, VP recognition alone is not sufficient for capsid assembly. Our study sheds light on the modular structure-function correlates of AAP and provides multiple approaches to engineer AAP that might prove useful toward understanding and controlling AAV capsid assembly.