Biophysical and Biochemical Studies of HIV-1 Capsid Interactions

Human immunodeficiency virus type 1 (HIV-1) is a virus of global public health concern that weakens a host’s immune system by attacking their immune cells, increasing susceptibility to other diseases and often leading to the development of acquired immune deficiency syndrome (AIDS). The HIV-1 capsid is a dynamic molecular machine essential to productive infection as it functions in numerous events in the viral life cycle. The mature capsid consists of ~1500 copies of the monomeric capsid (CA) protein arranged within an extended lattice that contains about 250 hexamers and exactly 12 pentamers to form a fullerene cone shell. Structural stability of the capsid lattice is crucial for proper completion of early infection events. Many binding partners of the capsid modulate lattice stability with either promotive or restrictive effects. In this work, we used a clean biochemical system to elucidate important details about the HIV-1 capsid and its binding partners, including host cell factor polyglutamine binding protein 1 (PQBP1). This intrinsically disordered protein bridges viral capsid recognition with the recruitment of cGAS (cyclic GMP-AMP synthase), a pattern recognition receptor that stimulates downstream induction of interferon upon detection of cytosolic double-stranded DNA. We provide structural evidence of PQBP1 binding to the capsid with the primary interaction occurring between the electronegative N-terminus of PQBP1 and the electropositive arginine-18 ring found within CA hexamers, though additional regions of PQBP1 may contribute to capsid binding. Furthermore, we demonstrate that PQBP1 binding promotes capsid lattice destabilization, likely as a result of PQBP1 self-association driven by C-terminal dimerization. Through structural determination of cross-linked CA pentamers bound to HIV-1 antagonist GS-CA1, we also demonstrate structural plasticity of the CA pentamer as evidenced by its hexamer-like conformation in complex with this ligand. All together, this dissertation reveals new and important details about the HIV-1 capsid and two of its many binding partners, allowing for a better overall understanding of capsid’s structural dynamics throughout infection.