Centrioles are cylindrical, ninefold symmetrical structures with peripheral triplet microtubules strictly required to template cilia and flagella. The highly conserved protein SAS-6 constitutes the center of the cartwheel assembly that scaffolds centrioles early in their biogenesis. We determined the x-ray structure of the amino-terminal domain of SAS-6 from zebrafish, and we show that recombinant SAS-6 self-associates in vitro into assemblies that resemble cartwheel centers. Point mutations are consistent with the notion that centriole formation in vivo depends on the interactions that define the self-assemblies observed here. Thus, these interactions are probably essential to the structural organization of cartwheel centers.

The ability of electrospray to propel large viruses into a mass spectrometer is established and is rationalized by analogy to the atmospheric transmission of the common cold. Much less clear is the fate of membrane-embedded molecular machines in the gas phase. Here we show that rotary adenosine triphosphatases (ATPases)/synthases from Thermus thermophilus and Enterococcus hirae can be maintained intact with membrane and soluble subunit interactions preserved in vacuum. Mass spectra reveal subunit stoichiometries and the identity of tightly bound lipids within the membrane rotors. Moreover, subcomplexes formed in solution and gas phases reveal the regulatory effects of nucleotide binding on both ATP hydrolysis and proton translocation. Consequently, we can link specific lipid and nucleotide binding with distinct regulatory roles.

Maltose-modified poly(propylene imine) (PPI) dendrimers were synthesized by reductive amination of unmodified second- to fifth-generation PPI dendrimers in the presence of excess maltose. The dendrimers were characterized by using (1)H NMR, (13)C NMR, and IR spectroscopies; laser-induced liquid beam ionization/desorption mass spectrometry; dynamic light scattering analyses; and polyelectrolyte titration. Their scaffolds have enhanced molecular rigidity and their outer spheres, at which two maltose units are bonded to the former primary amino groups on the surface, have hydrogen-bond-forming properties. Furthermore, the structural features reveal the presence of a dense shell. Experiments involving encapsulation (1-anilinonaphthalene-8-sulfonic acid) and biological properties (hemolysis and interactions with human serum albumin (HSA) and prion peptide 185-208) were performed to compare the modified with the unmodified dendrimers. These experiments gave the following results: 1) The modified dendrimers entrapped a low-molecular-weight fluorescent dye by means of a dendritic box effect, in contrast to the interfacial uptake characteristic of the unmodified PPI dendrimers. 2) Both low- and high-generation dendrimers containing maltose units showed markedly reduced toxicity. 3) The desirable features of bio-interactions depended on the generation of the dendrimer; they were retained after maltose substitution, but were now mainly governed by nonspecific hydrogen-bonding interactions involving the maltose units. The modified dendrimers interacted with HSA as strongly as the parent compounds and appeared to have potential use as antiprion agents. These improvements will initiate the development of the next platform of glycodendrimers in which apparently contrary properties can be combined, and this will enable, for example, therapeutic products such as more efficient and less toxic antiamyloid agents to be synthesized.

We present high resolution UV-photoelectron spectra of cold mass selected Cun-, Agn-, and Aun- with n=53-58. The observed electron density of states is not the expected simple electron shell structure, but is strongly influenced by electron-lattice interactions. Only Cu55- and Ag55- exhibit highly degenerate states. This is a direct consequence of their icosahedral symmetry, as is confirmed by density functional theory calculations. Neighboring sizes exhibit perturbed electronic structures, as they are formed by removal or addition of atoms to the icosahedron and therefore have lower symmetries. Gold clusters in the same size range show completely different spectra with almost no degeneracy, which indicates that they have structures of much lower symmetry. This behavior is related to strong relativistic bonding effects in gold, as demonstrated by ab initio calculations for Au55-.

F-type ATPases are highly conserved enzymes used primarily for the synthesis of ATP. Here we apply mass spectrometry to the F1FO-ATPase, isolated from spinach chloroplasts, and uncover multiple modifications in soluble and membrane subunits. Mass spectra of the intact ATPase define a stable lipid ‘plug’ in the FO complex and reveal the stoichiometry of nucleotide binding in the F1 head. Comparing complexes formed in solution from an untreated ATPase with one incubated with a phosphatase reveals that the dephosphorylated enzyme has reduced nucleotide occupancy and decreased stability. By contrasting chemical cross-linking of untreated and dephosphorylated forms we show that cross-links are retained between the head and base, but are significantly reduced in the head, stators and stalk. Conformational changes at the catalytic interface, evidenced by changes in cross-linking, provide a rationale for reduced nucleotide occupancy and highlight a role for phosphorylation in regulating nucleotide binding and stability of the chloroplast ATPase.

Multidrug resistance is a serious barrier to successful treatment of many human diseases, including cancer, wherein chemotherapeutics are exported from target cells by membrane-embedded pumps. The most prevalent of these pumps, the ATP-Binding Cassette transporter P-glycoprotein (P-gp), consists of two homologous halves each comprising one nucleotide-binding domain and six transmembrane helices. The transmembrane region encapsulates a hydrophobic cavity, accessed by portals in the membrane, that binds cytotoxic compounds as well as lipids and peptides. Here we use mass spectrometry (MS) to probe the intact P-gp small molecule-bound complex in a detergent micelle. Activation in the gas phase leads to formation of ions, largely devoid of detergent, yet retaining drug molecules as well as charged or zwitterionic lipids. Measuring the rates of lipid binding and calculating apparent K D values shows that up to six negatively charged diacylglycerides bind more favorably than zwitterionic lipids. Similar experiments confirm binding of cardiolipins and show that prior binding of the immunosuppressant and antifungal antibiotic cyclosporin A enhances subsequent binding of cardiolipin. Ion mobility MS reveals that P-gp exists in an equilibrium between different states, readily interconverted by ligand binding. Overall these MS results show how concerted small molecule binding leads to synergistic effects on binding affinities and conformations of a multidrug efflux pump.mass spectrometry from native state | real time substrate monitoring P -glycoprotein (P-gp) is an ATP-driven low-specificity efflux pump that plays an important role in the clearance of xenotoxins (1, 2). P-gp is also a member of the ATP-Binding Cassette (ABC) family of transporters and exports hydrophobic cytotoxic compounds as well as natural products, cyclic, and linear peptides (1, 3-5). Overexpression of P-gp in tumor cells impairs targeted drug delivery and is a major pitfall for chemotherapies. Small molecule substrates partition in the plasma membrane (6, 7), before binding in the internal hydrophobic cavity formed in the inward conformation of the pump (8). Export is then thought to proceed in an ATP-dependent way through conformational changes from the inward to the outward facing forms, evidenced by FRET spectroscopy (9). Recent highresolution structures of eukaryotic P-gp from mouse (8) and Caenorhabditis elegans (10) were obtained in inward conformations. Two prokaryotic homologs of P-gp [Sav1866 (11) and MsbA (12)] were captured crystallographically in outward-facing conformers, reflecting ATP-bound states, as well as two different inward states for MsbA. From these X-ray structures it is possible to build up a picture of P-gp, alternating between inward-and outward-facing conformations.Despite decades of careful biochemical studies (13), and recent insights from crystallography, many questions remain, however. Specifically it has not yet been possible to trap P-gp in an outwardfacing state or to show how substrate binding activates ATPase act...

Electrospray ionization mass spectrometry (ESI-MS) has evolved into a powerful adjunct for structural biology, helping to unravel the quaternary structure of protein complexes. Increasing interest has led to the study of ever larger multicomponent systems. Investigating these large complexes with ESI has meant that progressively more complicated mass spectra have been recorded. Correct assignment of these spectra is essential to maximize the information content available. Here we present a new assignment strategy and a supporting software package that allows the investigation of large heterogeneous systems, previously beyond the scope of full spectral assignment due to their complexity. The strategy involves two parts. The first includes a peak fitting routine to determine charge state distributions and consequently the masses of the various subcomplexes. The second module distinguishes between solution and gas phase products depending on their mass to charge ratio and assigns these charge states to different subunit combinations. These fitting and assignment routines contain many internal checks for consistency and reveal mass shifts, dependent upon desolvation conditions and small molecule binding. Using a rotary ATPase as a working example, we show how this assignment strategy is capable of determining the stoichiometry and interactions of the 8 different subunits within this 29-subunit assembly.

Size-selected, ligand-free gold clusters with diameters less than 2 nm can be routinely generated in the gas phase. The pronounced size dependence of their physical and chemical properties is one of their most important features. Surfacedeposited gold clusters are particularly interesting for applications in nanotechnology and heterogeneous catalysis. [1][2][3][4][5][6][7] One prerequisite for such applications is a detailed knowledge of the cluster structures.The extensive literature on theoretical studies of gold clusters has been surveyed in recent reviews. [8,9] In principle, quantum-chemical methods allow many properties of small gold clusters to be predicted with high accuracy. However, for larger clusters, the large number of possible isomers impedes the search for a global energy minimum. Often, only a small set of the possible structures can be considered, without any guarantee that the global minimum is included in the set. Consequently, progress in the intermediate size regime can only be made through the joint use of experiment and theory.In this fashion, the structures of small gold cluster anions and cations with up to 13 atoms have been inferred through a comparison of theoretical and experimental collision cross sections from ion-mobility measurements. [10,11] A remarkable finding in this study was that the 2D!3D structural transition for Au n À occurs at the surprisingly large cluster sizes of n = 11 and 12. This result was later confirmed through a comparison of photoelectron spectroscopy (PES) data with calculated density of states (DOS) curves.[12] For Au n À with n = 16-18 and 21-24, experimental and theoretical evidence for hollow cage-like structures has been reported. [13][14][15] Au 20 À possesses a tetrahedral structure, [16] which corresponds to a fragment of the face-centered cubic (fcc) structure of bulk gold; the cluster consists only of surface atoms and does not contain any inner atoms.It has been suggested, on the basis of quantum-chemical calculations, that medium-sized gold clusters, such as Au n with n = 32-35, [17,18] 42, [19] , and 50, [20] also have cage-like structures. In contrast, in a recent study, Au 32 À was assigned an amorphous, but dense, structure on the basis of a comparison of data from PES and the calculated DOS.[21] Low-symmetry "disordered" structures have also been proposed by Garzón et al. for Au 28 and Au 55 .[22] These results were supported by a combined PES and theoretical study by Häkkinen et al., which excluded high-symmetry structures for Au 55 À (whereas Ag 55 À and Cu 55 À have icosahedral structures).[23]The PE spectra of several Au n À clusters, notably for n = 14, 20, 34, and 58, show prominent band gaps, [24] reflecting the large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the corresponding neutral clusters. The associated valence-electron counts correlate with the sequence of jellium-shell closings obtained from a simple free-electron model invoking (volume-filling) spherical ...