Magnetically Oriented Phospholipid Bilayered Micelles for Structural Studies of Polypeptides. Does the Ideal Bicelle Exist?

✍ Scribed by Regitze R. Vold; R.Scott Prosser

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 126 KB
• Volume: 113
• Category: Article
• ISSN: 1064-1866
• DOI: 10.1006/jmrb.1996.0187

✦ Synopsis

The recent discovery of magnetically aligned phospho-Following the original observation (4, 6) of partial alignlipid bilayered micelles (1-3) has been met with enthusiasm, ment of several polypeptides in anisotropic DMPC/DHPC because these novel liquid crystalline systems appear exsolutions, a few more studies have been made that illustrate tremely promising as membrane mimetics for a variety of the potential of the phospholipid bicelles. Deuterium NMR biophysical investigations. In particular, aqueous solutions spectra have been obtained (7) of Leu-enkephalin (YGGFL) of binary mixtures of long-(L) and short-chain (S) phosphaaligned in DMPC/DHPC solution, and only a few months tidylcholines form a well-oriented discotic nematic phase ago, Sanders et al. (8) and Howard and Opella (9, 10) reover a reasonably wide range of temperature, concentration, ported the first 13 C and 15 N NMR spectra of oriented proteins pH, and phospholipid composition (3). Since these bicelles in positively aligned bicellar solution. are composed only of phospholipids, rather than mixtures

Bicelles possess yet another desirable property: when the of phospholipid and detergent, their discovery has stimulated DMPC/DHPC ratio, q, is reduced, the bicelle diameter is a great deal of interest in their use for nuclear magnetic correspondingly reduced. Below a certain threshold value, resonance studies of membrane-associated peptides and prothe bicelles are no longer able to maintain magnetic alignteins. NMR spectra of mixed micelles containing 1,2-dimyment, and consequently, an isotropic solution is observed. ristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-di-Vold et al. (11) have demonstrated that high-resolution, twohexanoyl-sn-glycero-3-phosphocholine (DHPC) were shown dimensional NMR spectra with secondary structure informa-(3) to be consistent with the notion of disk-shaped phosphotion can be obtained from membrane-associated peptides in lipid aggregates in which DMPC forms an L a -type bilayered an isotropic solution of DMPC/DHPC bicelles. If indeed the section surrounded by a rim of DHPC, which excludes the bicelles maintain their diskoid shape in isotropic solution, myristic acid chains from contact with water. Sanders and this is a significant development. It means that atomic resolu-Schwonek also reported that the DMPC/DHPC mixtures tion, three-dimensional structures can be obtained by modern form moderately viscous, optically clear solutions that orient NMR methods (12) for peptides and proteins in what spontaneously in strong magnetic fields over a wide range amounts to a true phospholipid bilayer, rather than in deterof total lipid concentration [typically 5-40% (w/w)]. As is gent micelles. usual for phospholipid bilayers, the planar micelles align

The main concern regarding the DMPC/DHPC bicelles is with the average bilayer normal, n V , perpendicular to the whether the bilayered morphology is retained in both anisomagnetic field B 0 ; i.e., the orientational order parameter is tropic and isotropic solution. Using 31 P, 13 C, and 2 H NMR negative. Of special importance is the finding that biological spectroscopy, Sanders and Schwonek (3) convincingly aractivity is maintained for certain enzymes (e.g., diacylglycgued that their magnetically oriented DMPC/DHPC bicelles erol kinase) in these bilayered micelles, which were given are disk-shaped with partitioning of the two phospholipids. the name ''bicelles'' (4). The subsequent discovery (5) that In particular, they observed distinct 31 P NMR resonances the magnetic alignment of the bicelles can be changed from for DMPC and DHPC, with DMPC resonating upfield from n V ⊥ B 0 to nV B 0 by addition of small amounts of selected DHPC as is predicted for bicelles with DMPC predominantly lanthanide ions, Eu 3/ , Er 3/ , Tm 3/ , and Yb 3/ , received further in the planar center and the DHPC sequestered along the attention, because the new orientation with positive orientarim. Unfortunately, chemical shifts are notoriously hard to tional order parameter induced by these lanthanide ions may quantify in oriented media, and we have consequently rebe more suitable for studies of predominantly a-helical corded 2 H quadrupole-echo NMR spectra of both DMPC and membrane-spanning proteins.

DHPC in two identical, macroscopically oriented DMPC/ DHPC bicellar samples containing, respectively, small amounts of chain-deuterated DMPC or DHPC. The question * To whom correspondence should be addressed.