Structure and Function Studies of Human Retrovirus Particle Assembly and Morphology

Abstract

The retrovirus Gag protein is the primary driver of virus particle assembly. The capsid (CA) domain is critical for protein-protein interactions that lead to Gag multimerization. The Gag protein interaction network defines critical aspects of the retroviral lifecycle at steps such as particle assembly and virus maturation. Particle morphologies among retroviral genera are distinct, with intriguing differences observed relative to that of human immunodeficiency virus type 1 (HIV-1), particularly those observed with HIV type 2 (HIV-2) and human T-cell leukemia virus type 1 (HTLV-1). The overarching hypothesis under investigation in this dissertation is that the retroviral CA domain of Gag encodes the primary determinants that impact immature particle morphology and help to explain the differences observed among immature and mature particle morphologies. To test this hypothesis, several sets of experiments were conducted. First, analysis of the HIV-2 CA domain led to the discovery of amino acid residues –including L19, A41, I152, K153, K157, N194, and D196 (which are conserved), along with G38 and N127 (which are non-conserved), that significantly diminished Gag multimerization and particle assembly. Second, studies directed at understanding the differences in immature particle morphology between HIV-1 and HIV-2 led to the discovery that HIV-2 CA has a structured helix 12 that can interact with helix 10 in the absence of the SP1 region of Gag. Third, site-directed mutagenesis of the HTLV-1 CA domain led to the discovery of amino acid residues that are important for Gag multimerization, particle assembly, and morphology (i.e., Q138, E142, Y144, F147, and R150), and provide evidence that the conformation of the CACTD is important for proper CA-CA interactions. Taken together, these observations provide new insights into retroviral particle assembly and aid efforts to discover novel therapeutic targets for intervention of human retroviral infections.