Structural Analysis of the Ectodomain of the Anti-Viral Protein BST-2

Abstract

Human BST-2/tetherin is a host factor that inhibits release of HIV-1, HIV-2, and SIV from the cell surface. Viruses can evade this inhibition through antagonistic viral protein interactions with BST-2. Structurally, full-length BST-2 consists of an N-terminal cytoplasmic domain, a transmembrane domain, an ectodomain, and a C-terminal membrane anchor. The N-terminal side of the ectodomain contains three cysteine residues; each can contribute to the formation of cysteine-linked dimers. We explored the ectodomain of BST-2 to further understand the flexibility of the protein as it relates to function. Recent cellular studies suggest BST-2 is flexible with regards to the dimerization and ability to function properly. However, X-ray crystallography suggests the ectodomain is rigid. Through limited proteolysis, molecular dynamics and small-angle x-ray scattering, we showed that the ectodomain of BST-2 is flexible. However, the flexibility of the membrane bound BST-2 and the interaction between the HIV-1 viral antagonist protein, Vpu, is still unknown. To investigate the flexibility and the interaction between BST-2 and Vpu, we are optimizing conditions for purifying and crystallizing the full-length BST-2, Vpu and the BST-2/Vpu complex. These studies will show how the innate immune system protein, BST2, interferes with viral budding.