Adenoviruses are nonenveloped, linear double stranded DNA viruses. Its genome is completely surrounded by an icosohedral protein capsid build up of three major proteins group which are hexon, penton base and knobbed fiber.
The length of adenovirus linear genome is between 26 and 45 Kbp and therefore allows the virus to theoretically carry 22 to 40 genes. However, bigger genome size does not change the fact that this virus is still an obligate intracellular parasite that fully depends on the host cell machinery to survive and replicate.
Generally, this linear virus genome is about 36 Kbp in size and consists of immediate early (E1A), early (E1-E4), intermediate and late genes (L1-L5) (Figure 1). Transcription of these genes can be divided into early and late phase, respectively, occurring before or after DNA replication.
Figure 1 Adenovirus genome
An interesting and yet advantageous feature of this viral genome is that it has a terminal 55 kDa protein associated with each of the 5' ends of the linear dsDNA. Such protein functions as primers in viral replication and ensures that the ends of the virus' linear genome are adequately replicated.
Adenovirus Replication
The replication strategy of the virus depends on the nature of its genome, since different groups of viruses classify under Baltimore system will undergo different replication cycles due to the nature of their genome. Adenovirus is classified as Class I dsDNA virus with the following replication cycle.
Replication Cycle of Adenovirus
1. Attachment
Uptake process of the virion particle by the host cell. Two stage process involving an initial interaction of the fibre protein with a range of cellular receptors, which include the MHC class I molecule and the coxsackievirus-adenovirus receptor. The penton base protein then binds to the integrin family of cell surface heterodimers allowing internalization via receptor-mediated endocytosis. Most cells express primary receptors for the adenovirus fibre coat protein, however internalisation is more selective.
2. Penetration
Phagocytosis of the whole adenovirus virion into host cell cytoplasm as a phagocytic vacuole. The toxic activity of the pentons is responsible for rupture of the phagocytic membrane and release the virion particle into the cytoplasm.
3. Uncoating
Follows an ordered sequence, first the pentons and capsids are digested by the host cell lysozyme, releasing a spherical, partially uncoated particle into the cytoplasm. The core migrates to the nucleus through nuclear pores with the help of cellular microtubules, whereupon it is converted into a virus DNA-cell histone complex.
Figure 2 Attachment, penetration and uncoating steps in adenovirus replication
Gene Expression
Prior to DNA replication, immediate early and early mRNAs are transcribed from the input DNA. Transcription of the Adenovirus genome is regulated by virus-encoded trans-acting regulatory factors (products of the immediate early genes regulate expression of the early genes).
Phase Gene transcribed Protein translated
Immediate early, E1A trans-acting transcriptional regulatory factor
Early E1B, E2A, E2B, E3, E4, Regulatory factors and some virion proteins
Late Late genes , mostly virion proteins eg capsid, pentons etc
Figure 3 Gene expression of adenovirus genome
DNA Replication
The steps involved in adenovirus DNA replication can be summarized as follows:
• First, the viral genome is coated with DBP.
• This protein reacts co-operatively with the cellular transcription factor, NFI which binds to a recognition site within the origin of replication, separated from the 1-18 bp core by a precisely defined spacer region.
• NFIII also binds at a specific recognition site between nucleotides 39 and 48.
• Protein-protein interactions, between NFI and pol, and pTP and NFIII help recruit the pTP-pol heterodimer into the preinitiation complex.
• Interaction between the heterodimer and specific base pairs 9 to 18 in the DNA sequence ensures correct positioning and the complex is further stabilized by interactions between the incoming pTP-pol and the genome-bound TP.
• DNA replication is then initiated by a protein priming mechanism in which a covalent linkage is formed between the alpha-phosphoryl group of the terminal residue, dCMP and the beta-hydroxyl group of a serine residue in pTP, a reaction catalysed by pol. This acts as a primer for synthesis of the nascent strand.
• Base pairing with the second GTA triplet of the template strand guides the synthesis of a pTP-trinucleotide, which then jumps back 3 bases, to base pair with the first triplet (also GTA) and synthesis then proceeds by displacing the non-template strand.
• NFI dissociates as the first nucleotide binds just prior to the initiation reaction. Dissociation of pTP from pol begins as the pTP-trinucleotide is formed and is almost complete by the time 7 nucleotides have been synthesized.
• NFIII dissociates as the replication binding fork passes through the NFIII binding site.
Figure 4 Adenovirus DNA Replication
Assembly of Virion Particles and Lysis of Host Cell
The late phase of the adenovirus life cycle is focused on producing sufficient quantities of structural protein to pack all the genetic material produced by DNA replication. Once the viral components have successfully been replicated the virus is assembled into its protein shells and released from the cell as a result of virally induced cell lysis.
Figure 5 Summary of adenovirus replication cycle
No comments:
Post a Comment