Retrovirus - Structure

Structure

Virions of retroviruses consist of enveloped particles about 100 nm in diameter. The virions also contain two identical single-stranded RNA molecules 7-10 kilobases (kb) in length. Although virions of different retroviruses do not have the same morphology or biology, all the virion components are very similar.

The main virion components are:

• Envelope: composed of lipids obtained from the host plasma membrane during budding process as well as glycoprotein encoded by the env gene.

• RNA: consists of a dimer RNA. It has a cap at the 5' end and a poly(A) tail at the 3' end. The RNA genome also has terminal noncoding regions, which are important in replication, and internal regions that encode virion proteins for gene expression. The 5' end includes four regions, which are R, U5, PBS, and L. R region is a short repeated sequence at each end of the genome during the reverse transcription in order to ensure correct end-to-end transfer in growing chain. U5, on the other hand, is a short unique sequence between R and PBS. PBS (primer binding site) consists of 18 bases complementary to 3' end of tRNA primer. L region is an untranslated leader region that gives signal for packaging of genome RNA. The 3' end includes 3 regions, which are PPT (polypurine tract), U3, and R. PPT is primer for plus-strand DNA synthesis during reverse transcription. U3 is a sequence between PPT and R, which has signal that provirus can use in transcription. R is the terminal repeated sequence at 3' end. The Nucleotide sequence at the 5'-reminus of the avian sarcoma virus genome was initially sequenced by J. Shine and A. P. Czernilofsky et al. (Proc. Natl. Acad. Sci. USA, Vol 75, pp 1473–1477, 1977) and the nucleotide sequence of an untranslated but conserved domain at the 3'-end of the avian sarcoma virus genome was initially published by A.P. Czernilofsky et al. (Nucleic Acuds Research, Vol. 8, pp 2967–2984, 1980).

• Proteins: consisting of gag proteins, protease (PR), pol proteins and env proteins. Gag proteins are major components of the viral capsid, which are about 2000-4000 copies per virion. Protease is expressed differently in different viruses. It functions in proteolytic cleavages during virion maturation to make mature gag and pol proteins. Pol proteins are responsible for synthesis of viral DNA and integration into host DNA after infection. Finally, env proteins play a role in association and entry of virion into the host cell. Possessing a functional copy of an env gene is what makes retroviruses distinct from retroelements. The env gene serves three distinct functions: enabling the retrovirus to enter/exit host cells through endosomal membrane trafficking, protection from the extracellular environment via the lipid bilayer, and the ability to enter cells. The ability of the retrovirus to bind to its target host cell using specific cell-surface receptors is given by the surface component (SU) of the env, while the ability of the retrovirus to enter the cell via membrane fusion is imparted by the membrane-anchored trans-membrane component (TM). Thus the env protein is what enables the retrovirus to be infectious.