Adeno-associated viruses (AAVs) are small, non-enveloped viruses that package a single-stranded (ss)DNA genome of 4.7 kilobases (kb) within their T = 1 icosahedral capsid. AAVs are replication-deficient viruses that require a helper virus to complete their life cycle. Recombinant (r)AAVs have been utilized as gene delivery vectors for decades in gene therapy applications. So far, six rAAV-based gene medicines have been approved by the US FDA. The 4.7 kb ssDNA genome of AAV encodes nine proteins, including three viral structural/capsid proteins, VP1, VP2, and VP3; four large nonstructural proteins (replication-related proteins), Rep78/68 and Rep52/40; and two small nonstructural proteins. The two nonstructured proteins are viral accessory proteins, namely the assembly associated protein (AAP) and membrane-associated accessory protein (MAAP). Although the accessory proteins are conserved within AAV serotypes, their functions are largely obscure. In this review, we focus on the expression strategy and functional properties of the small nonstructural proteins of AAVs.

Human bocavirus 1 (HBoV1) is a human parvovirus that causes lower respiratory tract infections in young children. It contains a single-stranded (ss) DNA genome of ~5.5 kb that encodes a small noncoding RNA of 140 nucleotides known as bocavirus-encoded small RNA (BocaSR), in addition to viral proteins. Here, we determined the secondary structure of BocaSR in vivo by using DMS-MaPseq. Our findings reveal that BocaSR undergoes N6-methyladenosine (m6A) modification at multiple sites, which is critical for viral DNA replication in both dividing HEK293 cells and nondividing cells of the human airway epithelium. Mechanistically, we found that m6A-modified BocaSR serves as a mediator for recruiting Y-family DNA repair DNA polymerase (Pol) η and Pol κ likely through a direct interaction between BocaSR and the viral DNA replication origin at the right terminus of the viral genome. Thus, this report represents direct involvement of a viral small noncoding RNA in viral DNA replication through m6A modification.

Recombinant adeno-associated viruses (rAAVs) are widely used for in vivo gene delivery. While KIAA0319L, known as AAV receptor (AAVR), is essential for the transduction of multiserotype AAVs, it is dispensable for AAV4-related (Clade G) AAVs. We conducted a genome-wide CRISPR/Cas9 screen and identified CD164, a type I transmembrane sialomucin, as an essential entry receptor for Clade G AAVs. Ablation of CD164 expression substantially impaired both entry and transduction of Clade G AAVs. CD164-targeting antibodies and soluble CD164 ectodomain effectively blocked transduction. AAV4 capsids colocalized with CD164 at the plasma membrane and in endosomal compartments. In vitro, CD164 interacted with AAV4 or AAVrh32.33 capsids at high affinity. Importantly, systemic administration of rAAV4 or rAAVrh32.33 in CD164 knockout (KO) mice resulted in nearly complete loss of transgene expression. These findings establish CD164 as an essential entry receptor for Clade G AAV vectors and uncover a distinct AAVR-independent mechanism of AAV tropism.

Parvovirus B19 (B19V), a member of the genus Erythroparvovirus within the Parvoviridae family, infects human erythroid progenitor cells (EPCs) of bone marrow and fetal liver and causes various hematological disorders. The minor capsid protein VP1 of B19V contains a unique N-terminal region (VP1u) that facilitates virus binding and internalization into EPCs via its receptor-binding domain (RBD). We previously identified tyrosine-protein kinase receptor UFO (AXL) as a proteinaceous receptor for B19V infection of EPCs. In this study, we employed an ascorbate peroxidase 2 (APEX2)-based proximity labeling method to identify host proteins that are associated with B19V VP1u during entry. This analysis revealed human transferrin receptor 1 (hTfR) as a key host protein associated with VP1u. hTfR knockdown in UT7/Epo-S1 cells, a B19V-permissive human megakaryoblastoid leukemia cell line, showed significantly reduced B19V internalization and replication. Biolayer interferometry (BLI) assays confirmed a direct interaction between B19V VP1u and hTfR extracellular domain (ECD). Inhibition of VP1u interaction with hTfR ECD, either by a monoclonal antibody targeting the apical domain of the ECD or human ferritin, a natural ligand of hTfR that binds the apical domain, significantly reduced VP1u binding to hTfR, as well as B19V internalization and B19V replication in ex vivo -expanded EPCs. Furthermore, mutant RBD proteins that bear amino acid substitutions in the three helical domains nearly abolished RBD binding to hTfR and significantly reduced the ability to inhibit B19V infection of EPCs. Collectively, our findings establish hTfR as a B19V entry co-receptor that mediates B19V internalization into its natural host EPCs. IMPORTANCE: B19V causes severe hematological disorders, including transient aplastic crisis, chronic pure red cell aplasia, and hydrops fetalis, by selectively infecting erythroid progenitor cells (EPCs). Despite its clinical impact, no approved antivirals or vaccines exist, largely due to limited understanding of viral entry mechanisms. A unique feature of B19V is the externalization of the VP1 unique region (VP1u) from the viral capsid, which mediates receptor engagement. Our prior studies identified AXL as a co-receptor for B19V infection of EPCs. Here, we identify that human transferrin receptor 1 (hTfR) as a critical co-receptor that directly binds VP1u and promotes viral internalization. Inhibition of the VP1u-hTfR interaction by competitive binding of hTfR with either an anti-hTfR monoclonal antibody or human ferritin significantly reduces B19V internalization and replication in ex vivo -expanded EPCs, highlighting a link between VP1u binding to the apical domain of hTfR and viral internalization. RBD mutants that disrupt its interaction with hTfR barely inhibited B19V infection in EPCs. These findings support a model in which hTfR directly engages VP1u and promotes B19V internalization into EPCs. Defining these mechanisms provides a foundation for developing antiviral strategies targeting B19V entry into EPCs.

The adeno-associated virus receptor (AAVR; also known as KIAA0319L) is the proteinaceous receptor required for the transduction of multiserotype adeno-associated viruses (AAVs). While the extracellular polycystic kidney disease (PKD) domains of AAVR directly bind AAV capsids, the function of its C-terminal cytosolic domain (AAVR-C) in governing AAV internalization and intracellular trafficking remains undefined. Using targeted pulldown of AAVR-interacting host proteins with a glutathione S-Transferase (GST)-fused AAVR-C recombinant protein (GST-AAVR-C) as a bait, we identified that TBC1 domain family member 23 (TBC1D23), a specialized intracellular trafficking adaptor and bridging factor, binds AAVR via the AAVR-C and mediates AAV’s endosome-to- trans -Golgi network (TGN) transport. Biolayer interferometry (BLI) demonstrates nanomolar-affinity binding between AAVR-C and the C-terminal scaffold domain of TBC1D23 (TBC1D23-C). CRISPR-mediated knockout of TBC1D23 severely impairs rAAV transduction across multiple human cell types, including polarized human airway epithelium (HAE). Loss of TBC1D23 disrupts convergence of internalized AAV capsids at the TGN, leading to diminished nuclear import of AAV vectors. Mutational analyses of AAVR-C reveal that its acidic residue cluster is required for TBC1D23 binding and AAV retrograde transport from the endosome to the TGN. Together, these findings define TBC1D23 as a receptor-proximal trafficking module that couples AAV–AAVR engagement to vesicle transport, revealing a missing core regulatory step in AAV retrograde transport that is essential for productive rAAV transduction.