The structure of PAS shows that the resting state of the key catalytic residue in sulfatases is a formylglycine hydrate. These structural data establish a mechanism for sulfate ester cleavage involving an aldehyde hydrate as the functional group that initiates the reaction through a nucleophilic attack on the sulfur atom in the substrate. The alcohol is eliminated from a reaction intermediate containing pentacoordinated sulfur. Subsequent elimination of the sulfate regenerates the aldehyde, which is again hydrated. The metal cation involved in stabilizing the charge and anchoring the substrate during catalysis is established as calcium.

Three point turn: A metallosupramolecular helicate with trigonal‐antiprismatic geometry fits perfectly into the central hydrophobic cavity of a three‐way DNA junction allowing a new mode of DNA recognition.

The amylose fraction of starch occurs in double-helical A-and B-amyloses and the single-helical V-amylose. The latter contains a channel-like central cavity that is able to include molecules, ''iodine's blue'' being the best-known representative. Molecular models of these amylose forms have been deduced by solid state 13 C cross-polarization͞magic angle spinning NMR and by x-ray fiber and electron diffraction combined with computer-aided modeling. They remain uncertain, however, as no structure at atomic resolution is available. We report here the crystal structure of a hydrated cycloamylose containing 26 glucose residues (cyclomaltohexaicosaose, CA26), which has been determined by real͞reciprocal space recycling starting from randomly positioned atoms or from an oriented diglucose fragment. This structure provides conclusive evidence for the structure of V-amylose, as the macrocycle of CA26 is folded into two short left-handed V-amylose helices in antiparallel arrangement and related by twofold rotational pseudosymmetry. In the V-helices, all glucose residues are in syn orientation, forming systematic interglucose O(3) n ⅐⅐⅐O(2) n؉l and O(6) n ⅐⅐⅐O (2) Starch is composed of two fractions, the linear amylose consisting exclusively of ␣(1-4)-linked glucose residues in 4 C 1 -chair conformation, and the branched amylopectin, which also contains ␣(1-6) links at characteristic intervals. The polysaccharide chain of amylose may be folded into three different structures denoted A, B, and V (1-3). Since crystallization of amylose fragments with defined chain lengths has remained elusive, structural information relies on x-ray fiber diffraction, electron diffraction on tiny single crystals, and solid-state 13 C cross-polarization͞magic angle spinning (CP͞MAS) NMR spectroscopy combined with computer-aided modeling (3-8). The only available single crystal x-ray study of an amylose-type oligosaccharide in the complex (p-nitrophenyl ␣-maltohexaoside) 2 ⅐Ba(I 3 ) 2 ⅐27H 2 O features an antiparallel left-handed double helix (9, 10) that has no resemblance to A-, B-, or V-amylose.The structures of A-and B-amylose are similar and differ only in packing arrangement and water content, the A-form occurring preferentially in cereals and the B-form in tubers (3). They both form double helices with parallel strands of 6 ϫ 2 glucoses per turn and right-handed (3-8) or left-handed (11) twist, this ambiguity illustrating the weakness of the above methods, which do not provide structural information at atomic resolution.The polysaccharide chain of V-amylose found naturally in non-A and non-B segments of amylose is folded into a lefthanded single helix; it contains 6 glucoses per turn with 7.91-to 8.17-Å pitch height (3-5) and forms a central channel-like cavity.

Ab initio macromolecular phasing has been so far limited to small proteins diffracting at atomic resolution (beyond 1.2 A) unless heavy atoms are present. We describe a general ab initio phasing method for 2 A data, based on combination of localizing model fragments such as small á-helices with Phaser and density modification with SHELXE. We implemented this approach in the program Arcimboldo to solve a 222-amino-acid structure at 1.95 A.

The meiotic chromosome axis plays key roles in meiotic chromosome organization and recombination, yet the underlying protein components of this structure are highly diverged. Here, we show that ‘axis core proteins’ from budding yeast (Red1), mammals (SYCP2/SYCP3), and plants (ASY3/ASY4) are evolutionarily related and play equivalent roles in chromosome axis assembly. We first identify ‘closure motifs’ in each complex that recruit meiotic HORMADs, the master regulators of meiotic recombination. We next find that axis core proteins form homotetrameric (Red1) or heterotetrameric (SYCP2:SYCP3 and ASY3:ASY4) coiled-coil assemblies that further oligomerize into micron-length filaments. Thus, the meiotic chromosome axis core in fungi, mammals, and plants shares a common molecular architecture, and likely also plays conserved roles in meiotic chromosome axis assembly and recombination control.

Recent advances in ab initio direct methods have enabled the solution of crystal structures of small proteins from native X-ray data alone, that is, without the use of fragments of known structure or the need to prepare heavy-atom or selenomethionine derivatives, provided that the data are available to atomic resolution. These methods are also proving to be useful for locating the selenium atoms or other anomalous scatterers in the multiple wavelength anomalous diffraction phasing of larger proteins at lower resolution.

Meiotic chromosomes adopt unique structures in which linear arrays of chromatin loops are bound together in homologous chromosome pairs by a supramolecular protein assembly, the synaptonemal complex. This three-dimensional scaffold provides the essential structural framework for genetic exchange by crossing over and subsequent homolog segregation. The core architecture of the synaptonemal complex is provided by SYCP1. Here we report the structure and self-assembly mechanism of human SYCP1 through X-ray crystallographic and biophysical studies. SYCP1 has an obligate tetrameric structure in which an N-terminal four-helical bundle bifurcates into two elongated C-terminal dimeric coiled-coils. This building block assembles into a zipper-like lattice through two self-assembly sites. N-terminal sites undergo cooperative head-to-head assembly in the midline, while C-terminal sites interact back to back on the chromosome axis. Our work reveals the underlying molecular structure of the synaptonemal complex in which SYCP1 self-assembly generates a supramolecular lattice that mediates meiotic chromosome synapsis.

SummaryLysR-type transcriptional regulators (LTTRs) constitute the largest family of regulators in prokaryotes. The full-length structures of the LTTR TsaR from Comamonas testosteroni T-2 and its complex with the natural inducer para-toluensulfonate have been characterized by X-ray diffraction. Both ligand-free and complexed forms reveal a dramatically different quaternary structure from that of CbnR from Ralstonia eutropha, or a putative LysR-type regulator from Pseudomonas aeruginosa, the only other determined full-length structures of tetrameric LTTRs. Although all three show a head-to-head tetrameric ring, TsaR displays an open conformation, whereas CbnR and PA01-PR present additional contacts in opposing C-terminal domains that close the ring. Such large differences may be due to a broader structural versatility than previously assumed or either, reflect the intrinsic flexibility of tetrameric LTTRs. On the grounds of the sliding dimer hypothesis of LTTR activation, we propose a structural model in which the closed structures could reflect the conformation of a ligand-free LTTR, whereas inducer binding would bring about local changes to disrupt the interface linking the two compact C-terminal domains. This could lead to a TsaR-like, open structure, where the pairs of recognition helices are closer to each other by more than 10 Å.