PISA wheel

PISA wheels and dipolar waves for oriented proteins... 

A very important aspect of solid-state NMR studies of uniaxiaUy oriented peptides is the possibility to directly determine the conformation of the protein in the bilayer. The PISA wheels 2 for regular secondary structures, which may be considered a direct NMR mapping of the so-called helical wheels, help interpreting the experimental spectra. More recently, Opella and co-workers have suggested to exclusively use the effective dipolar couplings in the analysis of such spectra to alleviate uncertainties from small residue/structure-specific variations in the chemical shifts. 

To illustrate the power of PISA wheels and dipolar waves to determine the structure of helical peptides and proteins in uniaxiaUy oriented lipid bilayers. Fig. 6a-c show SIMPSON/SIMMOL-simulated PISEMA spectra of an ideal 18-residue a-helix with a tilt angle of 10°-30° relative to Bq. In these simulations, we have tried to mimic experimental conditions by including a random distribution of the principal components of the chemical shift tensor and the dipolar coupling. The chemical shift distribution is 6 ppm for each principal element and has been established as follows we obtained — 85000 N isotropic chemical shifts reported to the BioMagResBank and selected only the — 31000 located in helical secondary stractures to have a data set independent on secondary chemical shifts. The standard deviation on the N chemical shifts for these resonances was — 6 ppm. With the lack of other statistically reliable experimental methods to establish such results for the individual principal elements of the N CSA tensor, we assumed the above variation of 6 ppm for all three principal elements. The variation of the H- N dipolar coupling was estimated by investigating the structures for a small number of a-helical membrane proteins for which the structures were established by liquid-state NMR spectroscopy. These showed standard deviations... 

Despite the fact that the simulated PISEMA spectra in Fig. 6a-c display signihcant fluctuations in the resonance positions relative to the ideal patterns, it is evident that both the PISA wheels and dipolar waves (represented by solid lines in Fig. 6) allow accurate determination of the helix tilt angle. The dipolar wave representation only shows minor differences between the resonance points and the ideal waves, and when trying to fit a dipolar wave to the resonance points, it does not display any visible difference to the ideal curve. 

Thus, for a given tilt angle r, the PISA wheel can be generated by rotating... 

Figure 8 depicts PISA wheels in which the helix tilt angle r varies from 15 to 90°. The magnitudes of the principal components of the 5-spin CSA tensor (533 = 64, 522 = 11, and 5n = 217 ppm) and the angles defining the relative orientations of the dipolar and chemical shift tensors (a = 0° and (1=17 ) were used in the simulations of PISA wheels presented in Fig. 8. The centers of the wheels as a function of helix tilt angle from both the peaks of the dipolar-coupling doublet are also shown as dashed lines in Fig. 8. The centers of the chemical shift and dipolar coupling tensors intersect at the isotropic values of the chemical shift (119.3 ppm) and the 4l " N dipolar coupling (0 Hz). The PISA wheel patterns can also be used to determine p-strand structures in lipid bilayers as the loop-like shapes of these p-strand resonances are very different from the wheel-like patterns of a-helices. It is clear from the...

Fig. 8. PISA wheel patterns for the helix tilt (r) varying from 15 to 90° simulated using Eq. (8) in the text. The chemical shift tensor values of 533 = 64, 522 = 77, and 5 =2I7 ppm, an N-H bond length of 1.07 A, and the relative orientations of the dipolar and chemical shift tensors of a = Q°, fi= T were used in the simulations. Variation of the PISA wheel with respect to the tilt angle of the helix is shown for one peak of the dipolar-coupling doublet as the spectrum is symmetric with respect to the zero frequency. The centers (shown in dashed lines) of the wheels as a function of the helix tilt angle for both dipolar transitions are linear and intersect at the isotropic N chemical shift frequency (119.3 ppm) and 0 Hz H °N dipolar coupling [Eq. (9)].

In general, 2D correlations of any two distinct, non-collinear, anisotropic spin interactions provide molecular images, like PISA wheels in the PISEMA spectrum. For proteins, spectra obtained from suitable combinations... 

Fig. 9. Two-dimensional PISEMA spectra of a SIMMOL-generated 300-residue polyalanine containing PISA wheels simulated using the SIMPSON program (A) an ideal a-helical peptide (0=—65°, 40°), (B) an ideal P-strand-I peptide...

While the PISA wheel patterns offer high throughput structure and... 

The PISEMA spectrum from uniformly labeled fd coat protein showed two PISA wheels (Fig. lOA and D), indicating two a-helices, one transmembrane helix with a tilt angle of 30 and another with an 87° tilt away from the bilayer normal.The PISA wheel patterns then can be simulated, and each helix is rotated around its axis separately until the resonance pattern in the calculated PISA wheel spectrum qualitatively matches the resonances in the experimental spectra of selectively labeled samples (Fig. lOB). From this procedure, both the rotation of the two helices and the sequential assignment can be obtained. In the next step, the chemical shift and dipolar coupling frequen-... 

D Correlation Spectroscopy. A simple, quahtative approach has been described for the determination of membrane protein secondary structure and topology in lipid bilayer membranes." The new approach is based on the observation of wheel-like resonance patterns in the NMR H- N/ N polarization inversion with spin exchange at the magic angle (PISEMA) and H/ N HETCOR spectra of membrane proteins in oriented lipid bilayers. These patterns, named Pisa wheels, have been previously shown to reflect helical wheel projections of residues that are characteristic of a-helices associated with membranes. This study extends the analysis of these patterns to P-strands associated with membranes and demonstrates that, as for the case of a-helices, Pisa wheels are extremely sensitive to the tilt, rotation, and twist of P-strands in the membrane and provide a sensitive, visually accessible, qualitative index of membrane protein secondary structure and topology. 

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