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 path ogenesis differ. Localization to the cytosolic leaflet of the plasma membra ne is critical for replication of both viruses. This localization is accomp lished 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-myristoylatio n signal. Interestingly, the MA protein of EIAV does not contain either of these signals. To understand which factors could promote EIAV assembly we c haracterized the membrane binding properties of its MA protein using fluore scence and biochemical methods. We find that EIAV MA exists as a multimer i n solution whose protein-protein interactions are destabilized by membrane binding. EIAV MA binds strongly to electrically neutral membranes as well a s to negatively charged membranes. Fluorescence quenching and chemical modi fication techniques, as well as trypsin proteolysis, indicate a different e xposure of the EIAV MA Trp residues when bound to the two types of membrane s, and EIAV MA proteolysis by trypsin differs when bound to the two types o f membranes. Based on these data and the known structures of closely relate d matrix proteins, we constructed a structural model. This model predicts t hat EIAV MA binds to negatively charged membranes, but EIAV MA has an addit ional membrane binding region rich in residues that partition favorably int o the membrane headgroup region. This secondary site may play a role in ear ly events of viral infection. (C) 2000 Academic Press.