BUSTER±TNT is a maximum-likelihood macromolecular re®nement package. BUSTER assembles the structural model, scales observed and calculated structure-factor amplitudes and computes the model likelihood, whilst TNT handles the stereochemistry and NCS restraints/constraints and shifts the atomic coordinates, B factors and occupancies. In real space, in addition to the traditional atomic and bulk-solvent models, BUSTER models the parts of the structure for which an atomic model is not yet available (`missing structure') as lowresolution probability distributions for the random positions of the missing atoms. In reciprocal space, the BUSTER structure-factor distribution in the complex plane is a twodimensional Gaussian centred around the structure factor calculated from the atomic, bulk-solvent and missing-structure models. The errors associated with these three structural components are added to compute the overall spread of the Gaussian. When the atomic model is very incomplete, modelling of the missing structure and the consistency of the BUSTER statistical model help structure building and completion because (i) the accuracy of the overall scale factors is increased, (ii) the bias affecting atomic model re®nement is reduced by accounting for some of the scattering from the missing structure, (iii) the addition of a spatial de®nition to the source of incompleteness improves on traditional Luzzati and ' A -based error models and (iv) the program can perform selective density modi®cation in the regions of unbuilt structure alone.

The complement system is a key component regulation influences susceptibility to age-related macular degeneration, meningitis, and kidney disease. Variation includes genomic rearrangements within the complement factor H-related ( CFHR ) locus. Elucidating the mechanism underlying these associations has been hindered by the lack of understanding of the biological role of CFHR proteins. Here we present unique structural data demonstrating that three of the CFHR proteins contain a shared dimerization motif and that this hitherto unrecognized structural property enables formation of both homodimers and heterodimers. Dimerization confers avidity for tissue-bound complement fragments and enables these proteins to efficiently compete with the physiological complement inhibitor, complement factor H (CFH), for ligand binding. Our data demonstrate that these CFHR proteins function as competitive antagonists of CFH to modulate complement activation in vivo and explain why variation in the CFHRs predisposes to disease.

Complement is an essential component of the innate and acquired immune system1, and consists of a series of proteolytic cascades that are initiated by the presence of micro-organisms. In health, activation of complement is precisely controlled through membrane-bound and soluble plasma-regulatory proteins including factor H (fH)2, a 155 kDa protein composed of twenty domains (termed complement control protein repeats, or CCPs). Many pathogens have evolved the ability to avoid immune- killing by recruiting host complement regulators3 and several pathogens have adapted to avoid complement-mediated killing by sequestering fH to their surface4. Here we present the first structure of a complement regulator in complex with its pathogen surface-protein ligand. This reveals how the important human pathogen Neisseria meningitidis subverts immune responses by mimicking the host, using protein instead of charged-carbohydrate chemistry to recruit the host complement regulator, factor H. The structure also indicates the molecular basis of the host-specificity of the interaction between factor H and the meningococcus, and informs attempts to develop novel therapeutics and vaccines.

Type III secretion systems (T3SSs) are bacterial membrane-embedded secretion nanomachines designed to export specifically targeted sets of proteins from the bacterial cytoplasm. Secretion through T3SS is governed by a subset of inner membrane proteins termed the ‘export apparatus’. We show that a key member of the Shigella flexneri export apparatus, MxiA, assembles into a ring essential for secretion in vivo. The ring forming interfaces are well conserved in both non-flagellar and flagellar homologues, implying that the ring is an evolutionary conserved feature in these systems. Electron cryo-tomography reveals a T3SS-associated cytoplasmic torus of size and shape corresponding to the MxiA ring aligned to the secretion channel located between the secretion pore and the ATPase complex. This defines the molecular architecture of the dominant component of the export apparatus and allows us to propose a model for the molecular mechanisms controlling secretion.