Molecular and Physicochemical Factors Governing Solubility of the HIV gp41 Ectodomain

The HIV gp41 ectodomain (e-gp41) is an attractive target for the development of vaccines and drugs against HIV because of its crucial role in viral fusion to the host cell. However, because of the high insolubility of e-gp41, most biophysical and structural analyses have relied on the production of truncated versions removing the loop region of gp41 or the utilization of nonphysiological solubilizing conditions. The loop region of gp41 is also known as principal immunodominant domain (PID) because of its high immunogenicity, and it is essential for gp41-mediated HIV fusion. In this study we identify the aggregation-prone regions of the amino acid sequence of the PID and engineer a highly soluble mutant that preserves the trimeric structure of the wild-type e-gp41 under physiological pH. Furthermore, using a reverse mutagenesis approach, we analyze the role of mutated amino acids upon the physicochemical factors that govern solubility of e-gp41. On this basis, we propose a molecular model for e-gp41 self-association, which can guide the production of soluble e-gp41 mutants for future biophysical analyses and biotechnological applications.