3-DIMENSIONAL STRUCTURE OF THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 MATRIX PROTEIN

The HIV-1 matrix protein forms an icosahedral shell associated with th e inner membrane of the mature virus. Genetic analyses have indicated that the protein performs important functions throughout the viral lif e-cycle, including anchoring the transmembrane envelope protein on the surface of the virus, assisting in viral penetration, transporting th e proviral integration complex across the nuclear envelope, and locali zing the assembling virion to the cell membrane. We now report the thr ee-dimensional structure of recombinant HIV-1 matrix protein, determin ed at high resolution by nuclear magnetic resonance (NMR) methods. The HIV-1 matrix protein is the first retroviral matrix protein to be cha racterized structurally and only the fourth HIV-1 protein of known str ucture. NMR signal assignments required recently developed triple-reso nance (H-1, C-13, N-15) NMR methodologies because signals for 91% of 1 32 assigned H-alpha protons and 74% of the 129 assignable backbone ami de protons resonate within chemical shift ranges of 0.8 p.p.m. and 1 p .p.m., respectively. A total of 636 nuclear Overhauser effect-derived distance restraints were employed for distance geometry-based structur e calculations, affording an average of 13.0 NMR-derived distance rest raints per residue for the experimentally constrained amino acids. An ensemble of 25 refined distance geometry structures with penalties (su m of the squares of the distance violations) of 0.32 Angstrom(2) or le ss and individual distance violations under 0.06 Angstrom was generate d; best-fit superposition of ordered backbone heavy atoms relative to mean atom positions afforded root-mean-square deviations of 0.50 (+/-0 .08) Angstrom. The folded HIV-1 matrix protein structure is composed o f five alpha-helices, a short 3(10) helical stretch, and a three-stran d mixed beta-sheet. Helices I to III and the 3(10) helix pack about a central helix (IV) to form a compact globular domain that is capped by the beta-sheet. The C-terminal helix (helix V) projects away from the beta-sheet to expose carboxyl-terminal residues essential for early s teps in the HIV-1 infectious cycle. Basic residues implicated in membr ane binding and nuclear localization functions cluster about an extrud ed cationic loop that connects beta-strands 1 and 2. The structure sug gests that both membrane binding and nuclear localization may be media ted by complex tertiary structures rather than simple linear determina nts.