Adsorption of the Human Immunodeficiency Virus Gag Polyprotein onto Lipid Membranes: A Study Using the Inner Field Compensation Method

The Gag polyprotein is the major structural protein of the human immunodeficiency virus (HIV). It is responsible for the assembly of new viral particles in the infected cell. This process takes place at the plasma membrane of the cell, and is largely regulated by the interactions of Gag with the lipid matrix of the cell membrane. In this work, we used the inner field compensation method and electrokinetic measurements of the zeta potential in a liposome suspension to study the binding of the non-myristoylated HIV Gag polyprotein to model lipid membranes. To quantify the affinity of the protein for charged and uncharged lipid bilayers, Gag adsorption isotherms were constructed and binding constants were calculated. It was shown that the protein is able to interact with both types of membranes with approximately the same intrinsic binding constants (K_PC = 8 × 10⁶ M^–1 and K_PS = 3 × 10⁶ M^–1). However, the presence of the anionic lipid phosphatidylserine in the lipid bilayer significantly enhances protein adsorption onto the membrane ( \(K_{{{\text{PS}}}}^{{{\text{eff}}}}\) = 37.2 × 10⁶ M^–1), because phosphatidylserine creates a surface potential jump near the membrane. Thus, the interaction of Gag with membranes is determined more by hydrophobic interactions and the area per lipid molecule, while the presence of a negative surface charge only increases the concentration of the positively charged protein near the membrane.