Molecular alignment of proteins in bicellar solutions: quantitative evaluation of effects induced in 2D COSY spectra

Inter-domain orientation and motions in VAT-N explored by residual dipolar couplings and 15N backbone relaxation

The N-terminal domain of VAT (Valosine-containing protein-like ATPase of Thermoplasma acidophilum), VAT-N (20.5 kDa), is considered to be the primary substrate-recognition site of the complex. The solution structure of VAT-N derived in our laboratory using conventionally obtained NMR restraints shows the existence of two equally sized sub-domains, VAT-Nn and VAT-Nc, together forming a kidney-shaped overall structure. The putative substrate-binding site of VAT-N involves free loops and a highly charged groove located on the surface of the protein. Alternatively, the opening of the cleft between the domains to accommodate substrate has been proposed to be part of the functional mechanism. We have used the residual dipolar couplings (RDCs) obtained in a bicelle medium to refine the structure of VAT-N. The long-range information available from RDCs both defines the sub-domain orientation and probes possible inter-domain motions. In addition, 15N backbone relaxation data were obtained and analysed within the model-free framework. Together, the data provides a refined structure with improved local geometry, but with the overall kidney shape intact. Further, the protein is rigid overall, with no evidence of inter-domain motions.

Investigating structural changes in the lipid bilayer upon insertion of the transmembrane domain of the membrane-bound protein phospholamban utilizing 31P and 2H solid-state NMR spectroscopy

Phospholamban (PLB) is a 52-amino acid integral membrane protein that regulates the flow of Ca(2+) ions in cardiac muscle cells. In the present study, the transmembrane domain of PLB (24-52) was incorporated into phospholipid bilayers prepared from 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC). Solid-state (31)P and (2)H NMR experiments were carried out to study the behavior of POPC bilayers in the presence of the hydrophobic peptide PLB at temperatures ranging from 30 degrees C to 60 degrees C. The PLB peptide concentration varied from 0 mol % to 6 mol % with respect to POPC. Solid-state (31)P NMR spectroscopy is a valuable technique to study the different phases formed by phospholipid membranes. (31)P NMR results suggest that the transmembrane protein phospholamban is incorporated successfully into the bilayer and the effects are observed in the lipid lamellar phase. Simulations of the (31)P NMR spectra were carried out to reveal the formation of different vesicle sizes upon PLB insertion. The bilayer vesicles fragmented into smaller sizes by increasing the concentration of PLB with respect to POPC. Finally, molecular order parameters (S(CD)) were calculated by performing (2)H solid-state NMR studies on deuterated POPC (sn-1 chain) phospholipid bilayers when the PLB peptide was inserted into the membrane.

Effects of small neutral molecules on phospholipid bicelle ordering

The effects of small neutral molecules on the liquid-crystalline ordering of dimyristoyl-phosphatidylcholine (DMPC)/dihexanoyl-phosphatidylcholine (DHPC) bicelles (q = 3.0 and 3.5) were studied via 2H, 31P, and 13C variable-temperature NMR. The addition of chloroform (up to approximately 90 mM, with a lipid concentration of approximately 120 mM) was observed to reduce the temperature onset of bicelle ordering by up to approximately 10 degrees C, likely resulting from the depression of the DMPC phase transition temperature. The temperature for the collapse of the bicelle phase was also significantly reduced; the observed effects amount to a downward shift in temperature (and reduction in range) of the liquid-crystalline portion of the bicelle phase diagram with increasing dopant concentration. Other model dopants (e.g., tetrahydrofuran and benzene) yielded smaller effects. Additionally, the variable bicelle alignment permitted the characterization of the ordering of chloroform molecules within the lipid phase.

Efficiency of homonuclear Hartmann-Hahn and COSY-type mixing sequences in the presence of scalar and residual dipolar couplings

In the presence of scalar (J) and residual dipolar (D) couplings, the transfer efficiency of homonuclear Hartmann-Hahn and COSY-type mixing depends on the ratio D/J and on the mixing sequence. This dependence is analyzed theoretically and the results are confirmed experimentally. At least two different mixing sequences are required to yield good transfer efficiencies for all ratios D/J. In contrast to COSY-type experiments, homonuclear Hartmann-Hahn sequences can provide efficient transfer even if the sum of D and J is zero, i.e., if the coupling vanishes in the weak coupling limit.