Barth van Rossum

Structure of a protein determined by solid-state magic-angle-spinning NMR spectroscopy

The determination of a representative set of protein structures is a chief aim in structural genomics. Solid-state NMR may have a crucial role in structural investigations of those proteins that do not easily form crystals or are not accessible to solution NMR, such as amyloid systems or membrane proteins. Here we present a protein structure determined by solid-state magic-angle-spinning (MAS) NMR. Almost complete 13C and 15N resonance assignments for a micro-crystalline preparation of the α-spectrin Src-homology 3 (SH3) domain formed the basis for the extraction of a set of distance restraints. These restraints were derived from proton-driven spin diffusion (PDSD) spectra of biosynthetically site-directed, labelled samples obtained from bacteria grown using [1,3-13C]glycerol or [2-13C]glycerol as carbon sources. This allowed the observation of long-range distance correlations up to ∼7 Å. The calculated global fold of the α-spectrin SH3 domain is based on 286 inter-residue 13C–13C and six 15N–15N restraints, all self-consistently obtained by solid-state MAS NMR. This MAS NMR procedure should be widely applicable to small membrane proteins that can be expressed in bacteria.

Backbone and Side-Chain 13C and 15N Signal Assignments of the α-Spectrin SH3 Domain by Magic Angle Spinning Solid-State NMR at 17.6 Tesla

The backbone and side‐chain 13C and 15N signals of a solid 62‐residue (u‐13C,15N)‐labelled protein containing the α‐spectrin SH3 domain were assigned by two‐dimensional (2D) magic angle spinning (MAS) 15N–13C and 13C–13C dipolar correlation spectroscopy at 17.6 T. The side‐chain signal sets of the individual amino acids were identified by 2D 13C–13C proton‐driven spin diffusion and dipolar recoupling experiments. Correlations to the respective backbone nitrogen signals were established by 2D NCACX (CX=any carbon atom) experiments, which contain a proton–nitrogen and a nitrogen–carbon cross‐polarisation step followed by a carbon–carbon homonuclear transfer unit. Interresidue correlations leading to sequence‐specific assignments were obtained from 2D NCOCX experiments. The assignment is nearly complete for the SH3 domain residues 7–61, while the signals of the N‐ and C‐terminal residues 1–6 and 62, respectively, outside the domain boundaries are not detected in our MAS spectra. The resolution observed in these spectra raises expectations that receptor‐bound protein ligands and slightly larger proteins (up to 20 kDa) can be readily assigned in the near future by using three‐dimensional versions of the applied or analogous techniques.

αB-Crystallin. A Hybrid Solid-State/Solution-State NMR Investigation Reveals Structural Aspects of the Heterogeneous Oligomer

Atomic-level structural information on alphaB-Crystallin (alphaB), a prominent member of the small heat-shock protein family, has been a challenge to obtain due its polydisperse oligomeric nature. We show that magic-angle spinning solid-state NMR can be used to obtain high-resolution information on an approximately 580-kDa human alphaB assembled from 175-residue 20-kDa subunits. An approximately 100-residue alpha-crystallin domain is common to all small heat-shock proteins, and solution-state NMR was performed on two different alpha-crystallin domain constructs isolated from alphaB. In vitro, the chaperone-like activities of full-length alphaB and the isolated alpha-crystallin domain are identical. Chemical shifts of the backbone and C(beta) resonances have been obtained for residues 64-162 (alpha-crystallin domain plus part of the C-terminus) in alphaB and the isolated alpha-crystallin domain by solid-state and solution-state NMR, respectively. Both sets of data strongly predict six beta-strands in the alpha-crystallin domain. A majority of residues in the alpha-crystallin domain have similar chemical shifts in both solid-state and solution-state, indicating similar structures for the domain in its isolated and oligomeric forms. Sites of intersubunit interaction are identified from chemical shift differences that cluster to specific regions of the alpha-crystallin domain. Multiple signals are observed for the resonances of M68 in the oligomer, identifying the region containing this residue as existing in heterogeneous environments within alphaB. Evidence for a novel dimerization motif in the human alpha-crystallin domain is obtained by a comparison of (i) solid-state and solution-state chemical shift data and (ii) (1)H-(15)N heteronuclear single quantum coherence spectra as a function of pH. The isolated alpha-crystallin domain undergoes a dimer-monomer transition over the pH range 7.5-6.8. This steep pH-dependent switch may be important for alphaB to function optimally (e.g., to preserve the filament integrity of cardiac muscle proteins such as actin and desmin during cardiac ischemia, which is accompanied by acidosis).

Large Protein Complexes with Extreme Rotational Correlation Times Investigated in Solution by Magic-Angle-Spinning NMR Spectroscopy

We show that large protein complexes can be investigated in solution using magic-angle-spinning (MAS) NMR spectroscopy without the need for sample crystallization or precipitation. In order to efficiently average anisotropic interactions with MAS, the rotational diffusion of the molecule has to be suppressed. This can be readily achieved by lowering the sample temperature and by adding glycerol to the protein solution. The approach is demonstrated using the human small heat shock protein (sHSP) alphaB-Crystallin, which forms oligomeric assemblies of approximately 600 kDa. We suggest this scheme as an approach for overcoming size limitations imposed by overall tumbling in solution-state NMR investigations of large protein complexes.

A MAS NMR study of the bacterial ABC transporter ArtMP

ATP‐binding cassette (ABC) transport systems facilitate the translocation of substances, like amino acids, across cell membranes energised by ATP hydrolysis. This work describes first structural studies on the ABC transporter ArtMP from Geobacillus stearothermophilus in native lipid environment by magic‐angle spinning NMR spectroscopy. The 2D crystals of ArtMP and 3D crystals of isolated ArtP were prepared in different nucleotide‐bound or ‐unbound states. From selectively 13C,15N‐labelled ArtP, several sequence‐specific assignments were obtained, most of which could be transferred to spectra of ArtMP. Residues Tyr133 and Pro134 protrude directly into the ATP‐binding pocket at the interface of the ArtP subunits, and hence, are sensitive monitors for structural changes during nucleotide binding and hydrolysis. Distinct sets of NMR shifts were obtained for ArtP with different phosphorylation states of the ligand. Indications were found for an asymmetric or inhomogeneous state of the ArtP dimer bound with triphosphorylated nucleotides. With this investigation, a model system was established for screening all functional states occurring in one ABC transporter in native lipid environment.