Retrovirus-Cell Interactions

Overall goals: Our research centers around understanding the interactions between viruses and host cells at the molecular level. We use retroviruses as a model system to dissect molecular mechanisms of virus replication, which has led us to study the intracellular trafficking pathways of retroviral proteins and cellular factors that are recruited to facilitate virus propagation.

Retrovirus replication: The main structural proteins of the oncogenic retrovirus Rous sarcoma virus (RSV) are the Gag proteins, which are initially synthesized as a polyprotein precursor. The Gag polyprotein directs the assembly and budding of progeny retrovirus particles from the plasma membrane of infected cells. Gag proteins are synthesized on free ribosomes in the cytosol, and previously it was believed that they were then targeted directly to the plasma membrane. However, recently we discovered that the RSV Gag protein actually enters the nucleus using signals in the MA and NC domains for nuclear targeting. Gag proteins are exported out of the nucleus through the nuclear pore complex via the cellular exportin CRM-1 pathway and an interaction with the nuclear pore complex proteins Nup98 and Nup214. After nuclear export, Gag proteins form multimeric complexes using viral RNA as scaffolding. The assembling virus particles are targeted to the plasma membrane where they interact with cellular machinery to allow budding of nascent virions.

Gag proteins and the nuclear compartment: One of our main goals is to understand what role Gag proteins play in the nucleus. We are testing two main hypotheses. First, the MA or NC proteins might be required early in infection, prior to the synthesis of the Gag polyprotein precursor. Alternatively, during virus assembly, the Gag polyprotein might enter the nucleus to interact with the viral RNA to initiate the genome encapsidation process. We are utilizing a multidisciplinary approach and a variety of techniques to test these hypotheses. We are interested in the dynamic intracellular trafficking pathways utilized by Gag proteins in their travel from cytosolic ribosomes to the nucleus, back to the cytoplasm and then to the plasma membrane. By dissecting the signals and factors involved in transport among subcellular compartments, our research interfaces with basic questions in cellular biology.