The human immunodeficiency virus type 1 (HIV-1) matrix protein forms a structural shell associated with the inner viral membrane and performs other essential functions throughout the viral life cycle. The crystal structure of the HIV-1 matrix protein, determined at 2.3 angstrom resolution, reveals that individual matrix molecules are composed of five major helices capped by a three-stranded mixed beta-sheet. Unexpectedly, the protein assembles into a trimer in three different crystal lattices, burying 1880 angstrom2 of accessible surface area at the trimer interfaces. Trimerization appears to create a large, bipartite membrane binding surface in which exposed basic residues could cooperate with the N-terminal myristoyl groups to anchor the protein on the acidic inner membrane of the virus.

Hill, C. P., Worthylake, D., Bancroft, D. P., Christensen, A. M., & Sundquist, W. I. (1996). Crystal structures of the trimeric human immunodeficiency virus type 1 matrix protein: implications for membrane association and assembly. Proceedings of the National Academy of Sciences, 93(7), 3099–3104.