A viral assembly factor promotes AAV2 capsid formation in the nucleolus
Open accessAbstract
The volume available in icosahedral virus capsids limits the size of viral genomes. To overcome this limitation, viruses have evolved strategies to increase their coding capacity by using more than one ORF while keeping the genome length constant. The assembly of virus capsids requires the coordinated interaction of a large number of subunits to generate a highly ordered structure in which the viral genome can be enclosed. To understand this process, it is essential to know which viral and nonviral components are involved in the assembly reaction. Here, we show that the adeno-associated virus (AAV) encodes a protein required for capsid formation by means of a nested, alternative ORF of the cap gene. Translation is initiated at a nonconventional translation start site, resulting in the expression of a protein with a calculated molecular weight of 23 kDa. This protein, designated assembly-activating protein (AAP), is localized in the host cell nucleolus, where AAV capsid morphogenesis occurs. AAP targets newly synthesized capsid proteins to this organelle and in addition fulfils a function in the assembly reaction itself. Sequence analysis suggests that also all other species of the genus Dependovirus encode a homologous protein in their cap gene. The arrangement of different ORFs that encode capsid proteins and an assembly factor within the same mRNA facilitates a timely coordinated expression of the components involved in the assembly process.
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All serotypes tested, with the exception of AAV4, AAV5 and AAV11, require AAP in order to assemble their capsid.
“In the most recent studies, it has been shown that all serotypes tested, with the exception of AAV4, AAV5 and AAV11, require AAP in order to assemble their capsid [35,39, 45–47].”
From Understanding Capsid Assembly and Genome Packaging for Adeno-Associated Viruses (2017) · cites this paper as [35]
The Results and Discussion show that AAP is required for AAV2 capsid assembly. The paper does not establish that all tested serotypes require AAP or identify AAV4, AAV5, and AAV11 as exceptions; its discussion of other serotypes concerns sequence conservation, not assembly requirements. (LLM, conf=0.88)
Experimental — accuracy not yet measuredAAP binds to AAV VP multimers across the twofold axis of symmetry at β strands located at the inner surface of the capsid and targets these VPs to the nucleus where it facilitates capsid assembly.
“AAP was reported to bind to the AAV VP multimers across the twofold axis of symmetry (thus, a dimer interface) at the β strands located at the inner surface of the capsid and target these VPs to the nucleus where it facilitates capsid assembly [35].”
From Understanding Capsid Assembly and Genome Packaging for Adeno-Associated Viruses (2017) · cites this paper as [35]
The Results (page unavailable) show that AAP directs VP3 to nucleoli and promotes capsid formation. The Discussion (page unavailable) says the mechanism of AAP-mediated assembly remains elusive; our copy does not establish that AAP binds VP multimers at the twofold-axis dimer interface or at β strands on the capsid’s inner surface. (LLM, conf=0.90)
Experimental — accuracy not yet measuredAAP functions as a scaffold protein that facilitates the localization of VP3 to the nucleus.
“The AAP is reported to function as a scaffold protein that facilitates the localization of VP3 to the nucleus [35].”
From Understanding Capsid Assembly and Genome Packaging for Adeno-Associated Viruses (2017) · cites this paper as [35]
The Results report that coexpressing AAP moves some VP3 into the nucleolus, a region within the nucleus. The Discussion says AAP *could* act as a scaffold during capsid assembly, but presents that as one possible mechanism, not an established function. The claim overstates the scaffold role and describes nucleolar targeting less precisely as nuclear localization. (LLM, conf=0.88)
Experimental — accuracy not yet measuredIn addition to VP3, the assembly-activating protein (AAP) is also essential for capsid assembly.
“However, in addition to VP3, the AAP is also essential for capsid assembly [35].”
From Understanding Capsid Assembly and Genome Packaging for Adeno-Associated Viruses (2017) · cites this paper as [35]
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- Authors
- Sonntag, F., Schmidt, K., & Kleinschmidt, J. A.
- Affiliations
- German Cancer Research Center (DE)
- Venue
- Proceedings of the National Academy of Sciences
- Publisher
- National Academy of Sciences
- Published
- 2010-05-17
- Volume
- 107
- Issue
- 22
- Pages
- 10220-10225
- ISSN
- 0027-8424 (ISSN-L)
- 1091-6490
- PMC
- PMC2890453
- OpenAlex
- W2010666698
- Cited by
- 392
- References
- 6
- Citations by year
- 2026: 10 · 2025: 23 · 2024: 38 · 2023: 36 · 2022: 29 · 2021: 37 · 2020: 27 · 2019: 28 · 2018: 25 · 2017: 27 · 2016: 11 · 2015: 30 · 2014: 18 · 2013: 14 · 2012: 21
- FWCI
- 9.36
- Type
- journal article
- Language
- en
- Open access
- green
- www.ncbi.nlm.nih.gov
- Locations
- Proceedings of the National Academy of Sciences (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- Funders
- Deutsches Krebsforschungszentrum
- European Molecular Biology Laboratory
- Deutsche Forschungsgemeinschaft
- Topics
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Genetics › Virus-based gene therapy research
- Physical Sciences › Environmental Science › Ecology › Bacteriophages and microbial interactions
- Health Sciences › Medicine › Epidemiology › Virology and Viral Diseases
- Keywords
- adeno-associated virus type 2, capsid assembly, nucleolus, cap gene
- MeSH terms
- Dependovirus, Amino Acid Sequence, Base Sequence, Capsid, Cell Line, Cell Nucleolus, DNA, Viral, Genes, Viral, Genetic Complementation Test, HeLa Cells, Humans, Molecular Sequence Data, Sequence Homology, Nucleic Acid, Gene Expression, Open Reading Frames, Sequence Homology, Amino Acid, DNA Primers, Virus Assembly, Capsid Proteins
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