The mitochondrial outer membrane-anchored monoamine oxidase (MAO) is a biochemically important flavoenzyme that catalyzes the deamination of biogenic and xenobiotic amines. Its two subtypes, MAOA and MAOB, are linked to several psychiatric disorders and therefore are interesting targets for drug design. To understand the relationship between structure and function of this enzyme, we extended our previous low-resolution rat MAOA structure to the high-resolution wild-type and G110A mutant human MAOA structures at 2.2 and 2.17 Å, respectively. The high-resolution MAOA structures are similar to those of rat MAOA and human MAOB, but different from the known structure of human MAOA [De Colibus L, et al. (2005) Proc Natl Acad Sci USA 102:12684 -12689], specifically regarding residues 108 -118 and 210 -216, which surround the substrate/inhibitor cavity. The results confirm that the inhibitor selectivity of MAOA and MAOB is caused by the structural differences arising from Ile-335 in MAOA vs. Tyr-326 in MAOB. The structures exhibit a C-terminal transmembrane helix with clear electron density, as is also seen in rat MAOA. Mutations on one residue of loop 108 -118, G110, which is far from the active center but close to the membrane surface, cause the solubilized enzyme to undergo a dramatic drop in activity, but have less effect when the enzyme is anchored in the membrane. These results suggest that the flexibility of loop 108 -118, facilitated by anchoring the enzyme into the membrane, is essential for controlling substrate access to the active site. We report on the observation of the structure-function relationship between a transmembrane helical anchor and an extra-membrane domain.single transmembrane helix ͉ transmembrane helical anchor ͉ harmine ͉ x-ray structure M onoamine oxidase (MAO) is an enzyme localized to the mitochondrial outer membrane and catalyzes the deamination of biogenic and xenobiotic amines, such as neuroactive serotonin, norepinephrine, and dopamine. MAO contains a flavin adenine dinucleotide (FAD) covalently bound to a cysteine residue by an 8␣-(S-cysteinyl)-riboflavin linkage (1). MAO plays a decisive role in some psychiatric and neurological disorders, including depression and Parkinson's disease. Because inhibition of MAO increases the level of neurotransmitters in the central nervous system, searching for the effective inhibitors represents one important approach to developing novel drugs to treat such illnesses. MAO has two subtypes, MAOA and MAOB, whose amino acid sequences are up to 70% identical, although each enzyme has unique substrate and inhibitor specificities (2): MAOA oxidizes serotonin, whereas MAOB does not; MAOA is selectively inhibited by clorgyline, whereas MAOB is highly inhibited by deprenyl. In addition, the oxidative deamination produces harmful hydrogen peroxide that may further generate free radicals (3). Development of selective and reversible MAO inhibitors is important not only from the standpoint of treating symptoms (i.e., by increasing the biological half-life of mo...

Hepatitis E virus (HEV) is a causative agent of acute hepatitis. The crystal structure of HEV-like particles (HEV-LP) consisting of capsid protein was determined at 3.5-Å resolution. The capsid protein exhibited a quite different folding at the protruding and middle domains from the members of the families of Caliciviridae and Tombusviridae, while the shell domain shared the common folding. Tyr-288 at the 5-fold axis plays key roles in the assembly of HEV-LP, and aromatic amino acid residues are well conserved among the structurally related viruses. Mutational analyses indicated that the protruding domain is involved in the binding to the cells susceptive to HEV infection and has some neutralization epitopes. These structural and biological findings are important for understanding the molecular mechanisms of assembly and entry of HEV and also provide clues in the development of preventive and prophylactic measures for hepatitis E.capsid ͉ HEV ͉ VLP

The OprM lipoprotein of Pseudomonas aeruginosa is a member of the MexAB-OprM xenobiotic-antibiotic transporter subunits that is assumed to serve as the drug discharge duct across the outer membrane. The channel structure must differ from that of the porintype open pore because the protein facilitates the exit of antibiotics but not the entry. For better understanding of the structure-function linkage of this important pump subunit, we studied the x-ray crystallographic structure of OprM at the 2.56-Å resolution. The overall structure exhibited trimeric assembly of the OprM monomer that consisted mainly of two domains: the membrane-anchoring ␤-barrel and the cavity-forming ␣-barrel. OprM anchors the outer membrane by two modes of membrane insertions. One is via the covalently attached NH 2 -terminal fatty acids and the other is the ␤-barrel structure consensus on the outer membrane-spanning proteins. The ␤-barrel had a pore opening with a diameter of about 6 -8 Å, which is not large enough to accommodate the exit of any antibiotics. The periplasmic ␣-barrel was about 100 Å long formed mainly by a bundle of ␣-helices that formed a solvent-filled cavity of about 25,000 Å 3 . The proximal end of the cavity was tightly sealed, thereby not permitting the entry of any molecule. The result of this structure was that the resting state of OprM had a small outer membrane pore and a tightly closed periplasmic end, which sounds plausible because the protein should not allow free access of antibiotics. However, these observations raised another unsolved problem about the mechanism of opening of the OprM cavity ends. The crystal structure offers possible mechanisms of pore opening and pump assembly.

Vaults are among the largest cytoplasmic ribonucleoprotein particles and are found in numerous eukaryotic species. Roles in multidrug resistance and innate immunity have been suggested, but the cellular function remains unclear. We have determined the x-ray structure of rat liver vault at 3.5 angstrom resolution and show that the cage structure consists of a dimer of half-vaults, with each half-vault comprising 39 identical major vault protein (MVP) chains. Each MVP monomer folds into 12 domains: nine structural repeat domains, a shoulder domain, a cap-helix domain, and a cap-ring domain. Interactions between the 42-turn-long cap-helix domains are key to stabilizing the particle. The shoulder domain is structurally similar to a core domain of stomatin, a lipid-raft component in erythrocytes and epithelial cells.