University of Missouri School of Medicine MU Health School of Medicine

Marc Johnson

Marc Johnson, Ph.D

Associate Professor
Email: marcjohnson@missouri.edu
Phone: (573) 882-1519

Fields of Interest

  • Retrovirus assembly

Education

  • Ph.D. 1999, Oregon State University

Research Statement

Retrovirus assembly
Retroviral assembly is an elegant biological process that involves multiple viral and cellular actors from diverse locations within the cell converging at defined assembly sites to create and facilitate the egress of an infectious viral particle. I utilize the model retrovirus HIV-1 to study how the basic viral components (proteins, glycoproteins, and RNAs) are targeted to viral assembly sites and how cellular processes are usurped to aid in this process. A combination of novel fluorescence and scanning electron microscopy techniques are utilized to better understand how these complex processes take place.

The focus of my research is understanding how viruses put themselves together. Most enveloped RNA viruses (which includes HIV, ebola, rabies, measles, and influenza, to name a few) are composed of the same basic ingredients: a viral RNA genome, viral structural proteins, and viral trans-membrane targeting/fusion glycoproteins. During an infection, these diverse viral components elegantly converge at a predetermined cellular location to assemble into a new virus that buds away from its host cell. This process is quite precise as most viruses contain a set number of viral genomes, a specific type of cellular lipid bi-layer, and very few cellular RNAs or proteins. Although researchers have made tremendous progress in recent years towards understanding what cellular processes are usurped by viruses to facility the process of viral release, very little is know about the early events that lead to viral assembly.

I primarily use retroviruses to study viral assembly and my work focuses on three model retroviruses: human immunodeficiency virus (HIV, the causative agent of AIDS), murine leukemia virus (MLV, the primary retroviral vector used in gene therapy), and Rous sarcoma virus (RSV, the classic model retrovirus). Retroviruses express a single structural protein, Gag, which by itself can assemble into non-infectious virus particles.

resarch 1


The assembly of Gag into a virus particle can be viewed at high magnifications in fixed cells using scanning or transmission electron microscopy (SEM and TEM) or at lower magnifications in live cells using fluorescently-tagged Gag proteins. I have developed a correlative system that combines both of these techniques by first imaging cells in real time by fluorescence microscopy and then imaging the exact same cells by SEM (Fig1), yielding a more comprehensive view of viral assembly. Using this technique I can begin to quantitatively dissect the pathway followed by the Gag as it assembles into a virus.

A second focus in viral assembly is understanding how viruses target their cytoplasmic structural proteins to the same assembly sites as their trans-membrane targeting/fusion glycoproteins. It has long been observed that enveloped viruses readily incorporate foreign viral glycoproteins, but exclude foreign cellular glycoproteins. This selective incorporation occurs even when the viral glycoproteins are entirely unrelated and contain no sequence similarity. These observations have led to the theory that viruses utilize a common glycoprotein targeting pathway/mechanism to drive viral incorporation. I have developed a method for observing viral assembly sites on a cell using standard secondary electron SEM while simultaneously viewing the distribution of gold labeled viral glycoproteins using backscatter SEM. Using this technique I have been able to show that there is a 50-100 fold enrichment of viral glycoproteins at viral assembly sites regardless of whether the structural proteins and glycoproteins are derived from the same virus (Fig 2A and B). By comparing similar proteins that are targeted to or excluded from budding sites, I now have a system to tease apart the requirements for and mechanisms of glycoprotein targeting.

resarch 2

Selected Publications

Lab Members

Terri
Terri Lyddon
Lab Manager
Devon
Devon Gregory
Post Doc
Sanath
Sanath Janaka
Graduate Student
Grace
Grace Olinger
Undergraduate Student
Jared
Jared Faurot
Undergraduate Student
Caroline
Caroline Hammond
Undergraduate Student

 

 

 

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