The Department of Biomedicine

BBB Seminar: Angela M. Gronenborn

Novel findings for HIV capsid function - Synergy between NMR, cryo-EM and large-scale MD simulations

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Angela M. Gronenborn
Department of Structural Biology and Pittsburgh Center for HIV Protein Interactions, University of Pittsburgh School of Medicine, PA, USA

Mature HIV-1 particles contain a conical-shaped capsid that encloses the viral RNA genome and performs essential functions in the virus life cycle. Previous structural analysis of two- and three-dimensional arrays provided a molecular model of the capsid protein (CA) hexamer and revealed three interfaces in the lattice. Using the high-resolution NMR structure of the CA C-terminal domain (CTD) dimer and in particular the unique interface identified, it was possible to reconstruct a model for a tubular assembly of the CA protein that fits extremely well into the cryo-electron microscopy (cryo-EM) density map. A novel CTD-CTD interface at the local three-fold axis in the cryo-EM map was confirmed by mutagenesis to be essential for function. More recently, the cryo-EM structure of the tube was solved at 8 Å resolution and this cryo-EM structure allowed unambiguous modeling and refinement by large-scale molecular dynamics (MD) simulation, resulting in all-atom models for the hexamer-of-hexamer and pentamer-of-hexamer elements of spheroidal capsids. Furthermore, the 3D structure of a native HIV-1 core was determined by cryo-electron tomography (cryo-ET), which, in combination with MD simulations, permitted the construction of a realistic all-atom model for the entire capsid, based on the 3D authentic core structure.

In addition, interaction with the innate immune defense restriction factor TRIM5α was studied. TRIM5α recognizes the lattice of the retrovirus capsid through its B30.2 (PRY/SPRY) domain in a species-specific manner. Upon binding, TRIM5α induces premature disassembly of the viral capsid and activates the downstream innate immune response. We have determined the crystal structure of the rhesus TRIM5α PRY/SPRY domain that reveals essential features for capsid binding. Combined cryo-EM and biochemical data show that the monomeric rhesus TRIM5α PRY/SPRY, but not human TRIM5α PRY/SPRY, can bind to HIV-1 capsid protein assemblies, without causing disruption of the capsid. Our data suggests a model for how this factor disrupts the virion core and suggests that structural damage to the viral capsid by TRIM5α is likely to be one of the important aspects of the mechanism of HIV-1 restriction.

Host: Aurora Martinez, Department of Biomedicine