Michael Sattler

Heteronuclear multidimensional NMR experiments for the structure determination of proteins in solution employing pulsed field gradients

Heteronuclear multidimensional NMR experiments for the structure determination of proteins in solution employing pulsed field gradients

A general enhancement scheme in heteronuclear multidimensional NMR employing pulsed field gradients.

SummaryGeneral pulse sequence elements that achieve sensitivity-enhanced coherence transfer from a heteronucleus to protons of arbitrary multiplicity are introduced. The building blocks are derived from the sensitivity-enhancement scheme introduced by Cavanagh et al. ((1991) J. Magn. Reson., 91, 429–436), which was used in conjunction with gradient coherence selection by Kay et al. ((1992) J. Am. Chem. Soc., 114, 10663–10665), as well as from a multiple-pulse sequence effecting a heteronuclear planar coupling Hamiltonian. The building blocks are incorporated into heteronuclear correlation experiments, in conjunction with coherence selection by the formation of a heteronuclear gradient echo. This allows for efficient water suppression without the need for water presaturation. The methods are demonstrated in HSQC-type experiments on a sample of a decapeptide in H2O. The novel pulse sequence elements can be incorporated into multidimensional experiments.

Structure and nucleic-acid binding of the Drosophila Argonaute 2 PAZ domain

RNA interference is a conserved mechanism that regulates gene expression in response to the presence of double-stranded (ds)RNAs. The RNase III-like enzyme Dicer first cleaves dsRNA into 21–23-nucleotide small interfering RNAs (siRNAs). In the effector step, the multimeric RNA-induced silencing complex (RISC) identifies messenger RNAs homologous to the siRNAs and promotes their degradation. The Argonaute 2 protein (Ago2) is a critical component of RISC. Both Argonaute and Dicer family proteins contain a common PAZ domain whose function is unknown. Here we present the three-dimensional nuclear magnetic resonance structure of the Drosophila melanogaster Ago2 PAZ domain. This domain adopts a nucleic-acid-binding fold that is stabilized by conserved hydrophobic residues. The nucleic-acid-binding patch is located in a cleft between the surface of a central β-barrel and a conserved module comprising strands β3, β4 and helix α3. Because critical structural residues and the binding surface are conserved, we suggest that PAZ domains in all members of the Argonaute and Dicer families adopt a similar fold with nucleic-acid binding function, and that this plays an important part in gene silencing.

Nucleic acid 3'-end recognition by the Argonaute2 PAZ domain.

We describe the solution structures of the Argonaute2 PAZ domain bound to RNA and DNA oligonucleotides. The structures reveal a unique mode of single-stranded nucleic acid binding in which the two 3′-terminal nucleotides are buried in a hydrophobic cleft. We propose that the PAZ domain contributes to the specific recognition of siRNAs by providing a binding pocket for their characteristic two-nucleotide 3′ overhangs.

SMN tudor domain structure and its interaction with the Sm proteins

Spinal muscular atrophy (SMA) is a common motor neuron disease that results from mutations in the Survival of Motor Neuron (SMN) gene. The SMN protein plays a crucial role in the assembly of spliceosomal uridine-rich small nuclear ribonucleoprotein (U snRNP) complexes via binding to the spliceosomal Sm core proteins. SMN contains a central Tudor domain that facilitates the SMN–Sm protein interaction. A SMA-causing point mutation (E134K) within the SMN Tudor domain prevents Sm binding. Here, we have determined the three-dimensional structure of the Tudor domain of human SMN. The structure exhibits a conserved negatively charged surface that is shown to interact with the C-terminal Arg and Gly-rich tails of Sm proteins. The E134K mutation does not disrupt the Tudor structure but affects the charge distribution within this binding site. An intriguing structural similarity between the Tudor domain and the Sm proteins suggests the presence of an additional binding interface that resembles that in hetero-oligomeric complexes of Sm proteins. Our data provide a structural basis for a molecular defect underlying SMA.

The Sand Domain Structure Defines a Novel DNA Binding Fold in Transcriptional Regulation

The SAND domain is a conserved sequence motif found in a number of nuclear proteins, including the Sp100 family and NUDR. These are thought to play important roles in chromatin-dependent transcriptional regulation and are linked to many diseases. We have determined the three-dimensional (3D) structure of the SAND domain from Sp100b. The structure represents a novel α/β fold, in which a conserved KDWK sequence motif is found within an α-helical, positively charged surface patch. For NUDR, the SAND domain is shown to be sufficient to mediate DNA binding. Using mutational analyses and chemical shift perturbation experiments, the DNA binding surface is mapped to the α-helical region encompassing the KDWK motif. The DNA binding activity of wild type and mutant proteins in vitro correlates with transcriptional regulation activity of full length NUDR in vivo. The evolutionarily conserved SAND domain defines a new DNA binding fold that is involved in chromatin-associated transcriptional regulation.

High-resolution X-ray and NMR Structures of the SMN Tudor Domain: Conformational Variation in the Binding Site for Symmetrically Dimethylated Arginine Residues

The SMN protein, which is linked to spinal muscular atrophy (SMA), plays an important role in the assembly of the spliceosomal small nuclear ribonucleoprotein complexes. This function requires binding of SMN to the arginine-glycine (RG) rich C-terminal tails of the Sm proteins, which contain symmetrically dimethylated arginine residues (sDMA) in vivo. Using NMR titrations, we show that the SMN Tudor domain recognizes these sDMAs in the methylated RG repeats. Upon complex formation a cluster of conserved aromatic residues in the SMN Tudor domain interacts with the sDMA methyl groups. We present two high resolution structures of the uncomplexed SMN Tudor domain, a 1.8A crystal structure and an NMR structure that has been refined against a large number of backbone and side-chain residual dipolar couplings. The backbone conformation of both structures is very similar, however, differences are observed for the cluster of conserved aromatic side-chains in the sDMA binding pocket. In order to validate these variations we introduce a novel application of residual dipolar couplings for aromatic rings. We show that structural information can be derived from aromatic ring residual dipolar couplings, even in the presence of internal motions such as ring flipping. These residual dipolar couplings and ring current shifts independently confirm that the SMN Tudor domain adopts two different conformations in the sDMA binding pocket. The observed structural variations may play a role for the recognition of sDMAs.

Determinants of conformational dimerization of Mad2 and its inhibition by p31comet

The spindle assembly checkpoint (SAC) monitors chromosome attachment to spindle microtubules. SAC proteins operate at kinetochores, scaffolds mediating chromosome‐microtubule attachment. The ubiquitous SAC constituents Mad1 and Mad2 are recruited to kinetochores in prometaphase. Mad2 sequesters Cdc20 to prevent its ability to mediate anaphase onset. Its function is counteracted by p31comet (formerly CMT2). Upon binding Cdc20, Mad2 changes its conformation from O‐Mad2 (Open) to C‐Mad2 (Closed). A Mad1‐bound C‐Mad2 template, to which O‐Mad2 binds prior to being converted into Cdc20‐bound C‐Mad2, assists this process. A molecular understanding of this prion‐like property of Mad2 is missing. We characterized the molecular determinants of the O‐Mad2:C‐Mad2 conformational dimer and derived a rationalization of the binding interface in terms of symmetric and asymmetric components. Mutation of individual interface residues abrogates the SAC in Saccharomyces cerevisiae. NMR chemical shift perturbations indicate that O‐Mad2 undergoes a major conformational rearrangement upon binding C‐Mad2, suggesting that dimerization facilitates the structural conversion of O‐Mad2 required to bind Cdc20. We also show that the negative effects of p31comet on the SAC are based on its competition with O‐Mad2 for C‐Mad2 binding.

Recognition of a Functional Peroxisome Type 1 Target by the Dynamic Import Receptor Pex5p

Peroxisomes require the translocation of folded and functional target proteins of various sizes across the peroxisomal membrane. We have investigated the structure and function of the principal import receptor Pex5p, which recognizes targets bearing a C-terminal peroxisomal targeting signal type 1. Crystal structures of the receptor in the presence and absence of a peroxisomal target, sterol carrier protein 2, reveal major structural changes from an open, snail-like conformation into a closed, circular conformation. These changes are caused by a long loop C terminal to the 7-fold tetratricopeptide repeat segments. Mutations in residues of this loop lead to defects in peroxisomal import in human fibroblasts. The structure of the receptor/cargo complex demonstrates that the primary receptor-binding site of the cargo is structurally and topologically autonomous, enabling the cargo to retain its native structure and function.

The three-dimensional structure of the HRDC domain and implications for the Werner and Bloom syndrome proteins.

BACKGROUND The HRDC (helicase and RNaseD C-terminal) domain is found at the C terminus of many RecQ helicases, including the human Werner and Bloom syndrome proteins. RecQ helicases have been shown to unwind DNA in an ATP-dependent manner. However, the specific functional roles of these proteins in DNA recombination and replication are not known. An HRDC domain exists in both of the human RecQ homologues that are implicated in human disease and may have an important role in their function. RESULTS We have determined the three-dimensional structure of the HRDC domain in the Saccharomyces cerevisiae RecQ helicase Sgs1p by nuclear magnetic resonance (NMR) spectroscopy. The structure resembles auxiliary domains in bacterial DNA helicases and other proteins that interact with nucleic acids. We show that a positively charged region on the surface of the Sgs1p HRDC domain can interact with DNA. Structural similarities to bacterial DNA helicases suggest that the HRDC domain functions as an auxiliary domain in RecQ helicases. Homology models of the Werner and Bloom HRDC domains show different surface properties when compared with Sgs1p. CONCLUSIONS The HRDC domain represents a structural scaffold that resembles auxiliary domains in proteins that are involved in nucleic acid metabolism. In Sgs1p, the HRDC domain could modulate the helicase function via auxiliary contacts to DNA. However, in the Werner and Bloom syndrome helicases the HRDC domain may have a role in their functional differences by mediating diverse molecular interactions.