From cell proteins to viral capsids

We have previously discussed the idea that viruses originated from selfish genetic elements such as plasmids and transposons when these nucleic acids acquired structural proteins (see A plasmid on the road to becoming a virus). I want to explore in more detail the idea that the structural proteins of  viruses likely originated from cell proteins (link to paper). Three ideas have emerged to explain the origin of viruses: 1. viruses evolved first on Earth, before cells, and when cells evolved, the viruses became their genetic parasites; 2. viruses are cells that lost many genes and became intracellular parasites; 3. viruses are collections of genes that escaped from cells. Missing from these hypothesis is how nucleic acids became virus particles – that is, how they acquired structural proteins. It seems likely that viral structural proteins originated from cellular genes. An analysis of the sequence an structure of major virion proteins has identified likely ancestors in cellular proteins. Following are some examples to illustrate this conclusion. A very common motif among viral capsid proteins is called the single jelly roll, made up of eight beta strands in two four-stranded sheets. Many cell proteins have jelly role motifs, and some form 60-subunit virus-like particles in cells. The extra sequences at the N-termini of viral jelly roll capsid proteins, involved in recognizing the viral genome, likely evolved after the capture of these proteins from cells. The core ...
Source: virology blog - Category: Virology Authors: Tags: Basic virology Information capsid evolution retrovirus selfish genetic element structural protein syncytin viral viruses Source Type: blogs