Nucleocytoplasmic transport of HIV proteins

HIV/AIDS afflicts over 30 million people worldwide. The advent of highly active anti-retroviral therapy (HAART) has reduced HIV/AIDS from a deadly disease to a manageable chronic infection. However, the long-term toxic effects of current anti-retroviral drugs and the rise in drug-resistant HIV variants means that there remains a strong need for new therapies that complement existing drugs.

Tat and Rev

An attractive anti-viral strategy is to block viral RNA transcription, a crucial stage in the HIV replication cycle. Key players in this stage are the HIV RNA-binding proteins Tat and Rev. Tat stimulates viral transcription, while Rev permits the nuclear export of unspliced transcripts encoding HIV structural proteins. Impairing the function of either protein severely compromises viral replication.

In order to function, Tat must enter the host cell nucleus, while Rev must shuttle between the nuclear and cytoplasmic compartments. These movements are mediated by Importin-β (Impβ) and CRM1, two host cell transport factors that deliver macromolecules (cellular and viral) across the nuclear envelope. Impβ binds its macromolecular cargo in the cytosol and releases it in the nucleus upon binding the small GTPase Ran. CRM1 associates co-operatively with its cargo and with Ran in the nucleus, forming a ternary complex that translocates to the cytosol and dissociates upon GTP hydrolysis.

Research goals

We aim to understand in atomic detail how Tat and Rev are recognized by Impβ and CRM1 during nuclear transport. A long-term goal is to exploit structural differences between cellular- and viral-cargo complexes to develop ligands that specifically inhibit (or stabilize) the latter. Such ligands could potentially interfere with the HIV infectious cycle by inhibiting viral cargo delivery to the nuclear or cytosolic compartments.

Nuclear transport of HIV Rev. Rev is recognized by Impβ in the cytosol and imported to the nucleus, where it is released by the binding of RanGTP. Rev then binds the Rev-response element (RRE), a specific RNA structure present in incompletely spliced viral transcripts. The Rev/RRE complex associates with CRM1 and RanGTP and is exported to the cytosol. Impβ also imports HIV Tat, which unlike Rev is not re-exported.

Pseudo-atomic model of CRM1 based on X-ray crystallography, homology modelling, and electron microscopy. The background shows electron microscopic images of single CRM1 molecules.