Biophysical Studies of HIV-1 Capsid Protein Assembly

Ayala Lampel, Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv, Israel
Or Berger, Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv, Israel
Oren Yaniv, Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv, Israel
Eran Bacharach, Cell Research and Immunology, Tel Aviv University, Tel Aviv, Israel
Felix Frolow, Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv, Israel
Ehud Gazit, Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv, Israel

Molecular self-assembly of small building blocks into ordered ultrastructures is one of the central themes in modern structural biology, supramolecular chemistry and nano-science. Virus assembly is unique, precise and highly specific process for viral infections. During this process, thousands of structural subunits interact through specific molecular recognition to form an intact functional particle. Even the slightest interference with the recognition between the building blocks, can result in aberrant assembly of a non functional particle. To gain new insights into the viral assembly process, we studied the kinetics of human immunodeficiency virus type 1 (HIV-1) capsid protein assembly in vitro. Our results support the notion that the assembly of HIV-1 capsid protein is a nucleation-dependent process displaying a rapid elongation step, and suggest the involvement of globular intermediate structures during the early steps of HIV-1 capsid assembly. In addition, we studied the effect of chemical chaperones on the assembly and stability of HIV-1 capsid protein. We found that while different groups of chemical chaperones markedly inhibited the protein assembly, other chemical chaperones dramatically enhanced the assembly rate. Compounds that inhibited the formation of capsid cores also stabilized the secondary structure of the protein under thermal denaturation. These findings indicate a correlation between the folded state of capsid protein to its tendency to assemble into high-ordered structures in vitro. Furthermore, we have determined the crystal structure of HIV-1 capsid C-terminal domain at 1.6 Å resolution, revealing a novel triclinic structure with two canonical dimers in the asymmetric unit. The canonical dimers form a newly identified packing interface, where interactions of four conserved residues take place. This is the first structural indication that these conserved residues participate in the putative CTD-CTD interactions. The findings suggest that our newly determined interface forms an assembly intermediate that participates in the early step of HIV-1 organization.