Visiting order on membrane proteins by using nanotechnology

During the past decade, there has been spectacular progress in membrane protein structural biology, with the number of determined structures increasing by an order of magnitude to more than 100 (1). Most structures of multispan integral membrane proteins (IMPs) have been determined by using x-ray crystallography. However, solution NMR methods have also provided a modest number of structures for both α-helical and β-barrel IMPs, ranging up to 30 kDa in protein molecular mass (2, 3). Moreover, momentum in both solution and solid-state NMR (4, 5) of IMPs is growing, thanks in part to iterative innovation between NMR pulse technology and sample preparation/manipulation. In the latter category is an important new method to achieve marginal alignment of membrane proteins, as introduced in this issue of PNAS by Douglas et al. (6). Marginal alignment is a prerequisite for experimentally accessing a rich source of NMR structural restraints: residual dipolar couplings (RDC). This novel method can be predicted to significantly extend the number of membrane proteins for which RDC can be accessed as well as to enhance the ease and accuracy by which RDC are measured.

Pairs of NMR-active nuclei in a magnetic field undergo through-space magnetic dipole–dipole interaction (“coupling”) that is experimentally manifested in the form of NMR peak splitting. The measured coupling constant is directly related to the time-averaged orientation of the internuclear vector with respect to the static magnetic field and is proportional to the inverse cube of the distance separating the coupled nuclei (Fig. 1). Because the internuclear distance is typically known in advance, it is the orientational dependence of dipolar couplings that makes these couplings a potent source of structural information. Indeed, there is growing consensus that high-quality 3D structures of bio-macromolecules can be determined from dipolar coupling data alone (7, 8). However, until approximately …

*E-mail: chuck.sanders{at}vanderbilt.edu