Properties of the Adeno-Associated Virus Assembly-Activating Protein

Abstract

b Adeno-associated virus (AAV) capsid assembly requires expression of the assembly-activating protein (AAP) together with capsid proteins VP1, VP2, and VP3. AAP is encoded by an alternative open reading frame of the cap gene. Sequence analysis and site-directed mutagenesis revealed that AAP contains two hydrophobic domains in the N-terminal part of the molecule that are essential for its assembly-promoting activity. Mutation of these sequences reduced the interaction of AAP with the capsid proteins. Deletions and a point mutation in the capsid protein C terminus also abolished capsid assembly and strongly reduced the interaction with AAP. Interpretation of these observations on a structural basis suggests an interaction of AAP with the VP C terminus, which forms the capsid protein interface at the 2-fold symmetry axis. This interpretation is supported by a decrease in the interaction of monoclonal antibody B1 with VP3 under nondenaturing conditions in the presence of AAP, indicative of steric hindrance of B1 binding to its C-terminal epitope by AAP. In addition, AAP forms high-molecular-weight oligomers and changes the conformation of nonassembled VP molecules as detected by conformation-sensitive monoclonal antibodies A20 and C37. Combined, these observations suggest a possible scaffolding activity of AAP in the AAV capsid assembly reaction.A deno-associated virus (AAV) is a nonenveloped singlestranded DNA virus of the Parvoviridae family (15). To date, 13 distinct human or nonhuman primate AAV serotypes have been described and numerous recombinant species have been isolated (10). The AAV assembly pathway proposed by Myers and Carter suggests the rapid formation of empty capsids into which the single-stranded genome is inserted in a slow reaction (16). While the process of genome replication has been elucidated in great detail (15,22), molecular events underlying capsid formation and genome encapsidation are less well understood (12).Capsid assembly occurs in the nuclei of infected cells, where capsids are first detectable in the nucleoli but are spread throughout the nucleus at later stages of infection (23). Expression of the cap gene is sufficient for capsid formation. Besides the three capsid proteins, VP1, VP2, and VP3, known to be expressed from open reading frame 1 (ORF1), the cap gene encodes an assembly factor, the assembly-activating protein (AAP), from a second ORF, ORF2 (21). AAP is essential for capsid assembly. It targets newly synthesized capsid proteins to the nucleolus and promotes capsid formation in a still unknown way. AAPs of some, but not all, AAV serotypes can cross-complement each other in the assembly reaction (20). AAP is a rather unstable protein but becomes stabilized upon the coexpression of capsid protein VP3. However, this stabilizing effect depends very much on the serotype of the coexpressed capsid protein, indicating specific AAP-VP protein interactions (20). AAP amino acid sequence alignment of serotypes 1 to 13 shows a high degree of homology. Only AAPs from serotypes 4, 5, 1...

Identification and Characterization of Nuclear and Nucleolar Localization Signals in the Adeno-Associated Virus Serotype 2 Assembly-Activating Protein

“…In keeping with this notion, mutations in this region could substantially reduce AAP2 protein levels without affecting its intracellular localization and could result in significantly impaired capsid assembly (AAP2mt8, -mt9, and -mt10). The reduction of capsid production by an AAP2BR1 mutation has also been reported by Naumer et al (1). Although the mechanism for the decreased protein expression has yet to be determined, it could be either due to intrinsic instability caused by protein misfolding or due to decreased protein-protein or protein-nucleic-acid interactions that stabilize AAP2.…”

“…Recently, a nonstructural viral protein encoded by an alternative ORF within the cap gene was identified and termed assembly-activating protein (AAP) for its indispensable role in capsid formation (1)(2)(3). The AAP (AAP2) from AAV serotype 2 (AAV2) is a nucleolar-localizing protein that binds to VP proteins through interacting domains in the amino (N) terminus of AAP2 (1), transports the VP proteins to the nucleolus, and promotes capsid assembly (3). Therefore, AAP2 is expected to have both a nuclear localization signal (NLS) and a nucleolar localization signal (NoLS) within its protein sequence.…”

Comparative Analysis of Adeno-Associated Virus Capsid Stability and Dynamics

“…A total of 60 copies of the three VPs, in a ratio of 1:1:8 to 10 (for VP1:VP2:VP3, respectively) based on gel densitometry (35,36), assemble the Tϭ1 icosahedral particle (37). A third alternative ORF, in the VP2/VP3 mRNA, which encodes an assembly-activating protein reported to aid capsid assembly, was recently discovered (38). It is believed that during cellular entry by AAV capsids, exposure to acidic conditions in endosomes leads to a conformational change and exposure of the VP1u and VP1/2 common regions required for endosomal escape and nuclear entry (34).…”

Establishment of a Novel Cell Line for the Enhanced Production of Recombinant Adeno-Associated Virus Vectors for Gene Therapy

“…Genotoxic or cytotoxic factors such as SV40 large T antigen (LTAg) that transformed or altered a cell cycle have also been shown to support AAV replication and assembly (Nicolas et al, 2012). It has been reported that total HeLa cell proteins and DNA helicase are required for AAV assembly (Steinbach et al, 1997;King et al, 2001;Sonntag et al, 2011;Naumer et al, 2012). A recent report using proteomics analysis of AAV vectors showed the association of 10 cellular proteins, for example, Nucleolin (NCL) and nucleophosmin (NPM1), with AAV vectors (Dong et al, 2014).…”