What Are VLPs?
VLPs are non-infectious virus-like particles that resemble the shell or capsid of a virus, but do not contain any viral genetic material. VLPs are a hot topic in research because of their many applications. VLPs also provide scientists with a safe way to research important but dangerous viruses. Our team is working with a VLP made from Gag polyproteins. The Gag polyprotein we are working with codes for a capsid, nucleocapsid, protease, reverse transcriptase, and integrase. All these pieces allow us to create VLPs.
Our VLP is derived from a centromeric retrotransposon found in Zea mays. Retrotransposons are segments of RNA that are known for their ability to “jump” to new places in an organism’s genome. Our centromeric retrotransposon is part of the Ty3-Gypsy superfamily. Our team has decided to work with centromeric retrotransposons for their unique ability to target and insert DNA in and near centromeres.
Our aim is to create a standardized VLP that would act as the perfect “box” to package genetic material or even small proteins. Because this particular Gag polyprotein is not well understood, we are striving to make contributions that will help other teams and researchers understand the nature of this polyprotein and the VLPs they create.
The first part of our project consists of conducting experiments to determine the conditions in which our protease actively cleaves the Gag polyprotein, and whether our Gag polyprotein self-assembles into VLPs in E. coli. Past studies suggest that Gag polyproteins from the Ty3-Gypsy family self-assemble into VLPs using E. coli as the chassis (1). By using electron microscopy, we can determine whether our VLP self-assembles in E. coli, or if in vitro methods are needed to create VLPs.
The second aim of our project is to create a system to verify whether VLPs have been formed without complete dependence on electron microscopy. Using a split-protein system with fluorescent proteins or ß-galactosidase would allow us to verify VLP formation. Attaching two halves of GFP to our Gag polyprotein so that these halves only connect when the VLP is fully formed would allow us to qualitatively verify that we have achieved VLP formation, without the use of electron microscopy. This is an important step in creating a VLP that is easy to use.
Our final challenge is creating a VLP that can efficiently package nucleic acids and proteins. Past studies show it is possible to package proteins within VLPs using a clever RNA aptamer, or a short RNA sequence that binds to the targeted molecule (2). Working to elucidate the RNA sequence our nucleocapsid binds to will allow us to create our own aptamer in which we can then have a standardized way to package targeted molecules.
Our hope for this project is that we create an easy-to-use, standardized VLP that can efficiently package nucleic acids and proteins. A standardized VLP could allow us and future iGEM teams to package a variety of molecules without having to alter the VLP itself.
Real World Applications
Since our VLP is derived from a centromeric retrotransposon, it has the ability to target the centromeres of our organism. Many plant transformation and gene transfer methods have been highly studied and continue to be improved. Gene transfer in many plants at efficient enough rates continues to be a difficult issue to tackle (3). Using our VLP to deliver genetic material to the centromere of the targeted plant could result in high retention of our inserted DNA through many generations, since DNA in and near centromeres are highly conserved in many organisms.
1. Larsen LSZ, Kuznetsov Y, McPherson A, Hatfield GW, Sandmeyer S. TY3 GAG3 protein forms ordered particles in Escherichia coli. Virology. 2008;370(2):223-227. doi:10.1016/j.virol.2007.09.017
2. Fiedler JD, Brown SD, Lau JL, Finn MG. RNA-Directed Packaging of Enzymes within Virus-like Particles **. Angew Chemie Int Ed. 2010;49:9648-9651. doi:10.1002/anie.201005243
3. Potrykus I. Gene Transfer to Plants: Assessment of Published Approaches and Results. Assessment. 1991:205-225. doi:10.1146/annurev.pp.42.060191.001225