The development of serial crystallography has been driven by the sample requirements imposed by X-ray free-electron lasers. Serial techniques are now being exploited at synchrotrons. Using a fixed-target approach to highthroughput serial sampling, it is demonstrated that high-quality data can be collected from myoglobin crystals, allowing room-temperature, low-dose structure determination. The combination of fixed-target arrays and a fast, accurate translation system allows high-throughput serial data collection at high hit rates and with low sample consumption.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease linked to the misfolding of Cu/Zn superoxide dismutase (SOD1).ALS-related defects in SOD1 result in a gain of toxic function that coincides with aberrant oligomerization. The structural events triggering oligomerization have remained enigmatic, however, as is the case in other protein-misfolding diseases. Here, we target the critical conformational change that defines the earliest step toward aggregation. Using nuclear spin relaxation dispersion experiments, we identified a short-lived (0.4 ms) and weakly populated (0.7%) conformation of metal-depleted SOD1 that triggers aberrant oligomerization. This excited state emanates from the folded ground state and is suppressed by metal binding, but is present in both the disulfide-oxidized and disulfide-reduced forms of the protein. Our results pinpoint a perturbed region of the excited-state structure that forms intermolecular contacts in the earliest nonnative dimer/ oligomer. The conformational transition that triggers oligomerization is a common feature of WT SOD1 and ALS-associated mutants that have widely different physicochemical properties. But compared with WT SOD1, the mutants have enhanced structural distortions in their excited states, and in some cases slightly higher excited-state populations and lower kinetic barriers, implying increased susceptibility to oligomerization. Our results provide a unified picture that highlights both (i) a common denominator among different SOD1 variants that may explain why diverse mutations cause the same disease, and (ii) a structural basis that may aid in understanding how different mutations affect disease propensity and progression.amyotrophic lateral sclerosis ͉ nuclear magnetic resonance relaxation ͉ protein misfolding

Proteins fold into their functional three-dimensional structure based on the information encoded in the residue sequence 1 . In all domains of life, various strategies evolved to assist folding processes in order to prevent immediate misfolding and aggregation of the nascent polypeptide chain. In the crowded cellular environment, protein folding is often aided by molecular chaperones. Molecular chaperones differ in size, function and energy dependence; however, all have in common to bind to the unfolded state of the protein in order to facilitate or mediate assembly of the correct three-dimensional structure 2,3 . In contrast to molecular chaperones, the intramolecular chaperones (IMCs) constitute a different class of chaperones. As part of the polypeptide chain, the IMC is typically cleaved off the target protein after the folding process is completed. Two classes of IMCs can be distinguished: class I IMCs assist the protein to fold into the correct tertiary structure, whereas class II IMCs are involved in quaternary structure assembly 4 . In contrast to many molecular chaperones, no evidence for an ATP-driven cleavage reaction could be found in IMCs. An example of a class II intramolecular chaperone has been identified in viral tailspike and fiber proteins. These proteins are functionally unrelated but share a highly conserved chaperone domain at their C terminus (C-terminal intramolecular chaperone domain, CIMCD), which is cleaved at a conserved position in an autoproteolytic reaction 5 . It was shown that the covalent linkage between the CIMCD and N-terminal pre-protein is necessary for correct folding, indicating that an in trans function of the chaperone is impossible 5 . Furthermore, it could be shown that some of the chaperone domains are exchangeable between the different preproteins 5,6 . While the three-dimensional structures of the CIMCDs are as yet unknown, the crystal structure of N-terminally truncated mature endoNF shows that the homotrimeric enzyme comprises a triple β-helix involved in substrate recognition 7 . This triple β-helix and the related triple-β-spiral motif have been identified in various proteins, which often play a role as virulence factors 8 . In triple β-helices, three polypeptide chains wind around a common threefold symmetry axis, conferring an extraordinary stability to the protein. The rigid elongated shape allows triple β-helix-comprising proteins to protrude from a pathogen's surface in order to interact with flexible host cell receptors, like lipopolysaccharides 9 . However, proper assembly of triple-β-helical folds poses to be difficult in the absence of a trimerization domain 10 . Hence, most triple β-helices depend on a C-terminal extension for trimerization and correct assembly 11 .Here we present the crystal structures of two representatives of a large group of systematically, functionally and structurally similar intramolecular chaperones: the Escherichia coli phage K1F endosilidase CIMCD and the Bacillus subtilis phage GA-1 neck appendage protein CIMCD. Furthermore...

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