The 51 serotypes of human adenovirus are divided into six species (A–F).¹² The human adenovirus serotypes 2 and 5 (Ad2 and Ad5, respectively), both from species C, have been most extensively characterized and have been used recently for construction of viral vectors for gene therapy. (See the later section, Adenovirus-Mediated Gene Transfer.) Ad2 and Ad5 genomes have been completely sequenced and are 95% identical at the nucleotide level, with a similar organization of transcriptional units (reviewed in Berk¹⁶). The Ad5 genome is a linear double-stranded DNA of approximately 36 kilobases (kb), with at least 30 different messenger ribonucleic acids (mRNAs). The mRNAs may be grouped into early and late transcripts, according to whether they are expressed before or after viral DNA replication (Fig. 96-1). Early transcripts (E1A, E1B, E3, and E4) encode proteins that modulate the host cell cycle, mediate viral DNA replication, and control the host immune response to viral invasion.²⁴ Three delayed early transcription units (IX, IVa2, and E2 late) and five late transcripts (termed L1 through L5) transcribe RNA polymerase II and encode viral structural proteins such as hexon, penton base, and fiber. The L1 to L5 transcripts are derived from this common transcript through alternative splicing. Other small transcripts that have important functions in the viral life cycle encode pIX (a structural protein), IVa2 (a protein that activates the MLP), and VA RNA I and II (RNAs that block activation of the host interferon response).

Adenoviruses are nonenveloped viruses. The viral structural proteins are DNA associated or involved in formation of the complex capsid encapsulating the viral DNA. The DNA-associated proteins include the terminal protein (TP) that binds to the 5′ termini of the viral genome containing the inverted terminal repeats (ITRs).²⁵ Other core proteins include polypeptides V and VII and protein X. Like cellular histones, these proteins are rich in the basic amino acid arginine, but unlike histones they contain appreciable amounts of tryptophan.^26,27 The precise function of each of the core proteins has yet to be elucidated, but protein V may play a role in interfering with host cellular function by redistributing nucleolar antigens.²⁸

The icosahedral capsid structure of adenovirus is made up primarily of three proteins: hexon, penton base, and fiber as shown in Figure 96-2. The capsid is formed from 252 unit structures, known as capsomeres, arranged in an icosahedral structure with 20 triangular sides. Hexagonal capsomeres or hexons constitute 240 of the structural subunits.²⁹ Each hexon is composed of three different polypeptides: VI, VII, and IX. There are also 12 pentagonal capsomeres or pentons. Each penton is composed of an 85-kilodalton (kDa) protein, polypeptide III. Polypeptide III is the largest protein of the entire viral capsid. Projecting from each penton is a rodlike structure called a fiber made up of three molecules of protein IV, a 62-kDa polypeptide.³⁰ The fiber and penton base not only contribute to the capsid structure but also are intimately involved in the attachment of the viral particle to the host cell surface.

The replication cycle of adenovirus may be divided into three stages: host infection, DNA replication, and construction of new viral particles. The fiber protein of the viral capsid attaches to a primary receptor on the host cell surface, much like an anchor. Three types of host cell-surface molecules can act as the primary receptor: the coxsackievirus group B and adenovirus receptor (CAR),³¹ the major histocompatibility complex (MHC)-I α2 subunit,³² and sialic acid residues on glycoprotein.³³ Once the viral particle is tethered through the primary interaction, a second interaction between the penton base and a secondary cell-surface receptor, the integrins α_vβ₃ and α_vβ₅, takes place. The integrins, in turn, promote internalization of the viral particle through receptor-mediated endocytosis.³⁴ Once inside the cell, the outer capsid protein is partially disassembled because of acidification of the contents of the endosome. This allows the virus to escape the endosome before the formation of the lysosome^35,36 and enter the cytoplasm. The process of viral internalization and release to the cytosol takes about 15 minutes.³⁷ Virus particles are then translocated to the nucleus along the microtubule network.³⁸ The adenoviral particle undergoes further disassembly, and, as a final step, the Ad hexon proteins remain at the nuclear membrane while the DNA is released into the nucleus.³⁷