Binding of equine infectious anemia virus matrix protein to membrane bilayers involves multiple interactions

Human immunodeficiency virus (HIV) and equine infectious anemia virus (EIAV) are closely related lentiviruses that infect immune cells, but their pathogenesis differ. Localization to the cytosolic leaflet of the plasma membrane is critical for replication of both viruses. This localization is accomplished through the matrix (MA) domain of the Gag precursor protein. In HIV-1, association of MA to anionic membranes appears to be primarily driven by a linear cluster of basic residues in the MA domain and an N-myristoylation signal. Interestingly, the MA protein of EIAV does not contain either of these signals. To understand which factors could promote EIAV assembly we characterized the membrane binding properties of its MA protein using fluorescence and biochemical methods. We find that EIAV MA exists as a multimer in solution whose protein-protein interactions are destabilized by membrane binding. EIAV MA binds strongly to electrically neutral membranes as well as to negatively charged membranes. Fluorescence quenching and chemical modification techniques, as well as trypsin proteolysis, indicate a different exposure of the EIAV MA Trp residues when bound to the two types of membranes, and EIAV MA proteolysis by trypsin differs when bound to the two types of membranes. Based on these data and the known structures of closely related matrix proteins, we constructed a structural model. This model predicts that EIAV MA binds to negatively charged membranes, but EIAV MA has an additional membrane binding region rich in residues that partition favorably into the membrane headgroup region. This secondary site may play a role in early events of viral infection.