Helical structure and dynamics in membrane polypeptides.

Abstract

Direct determination of the conformational properties of polypeptides and proteins reconstituted in phospholipid bilayer membranes is not easily achieved, since the main high-resolution structural methods (X-ray crystallography and high-resolution NMR spectroscopy) are rarely applicable to membrane systems. It is expected that solid-state NMR methods will make increasing contributions in this area [l], although some methods of sample preparation (e.g. oriented bilayers obtainable only at low hydration levels) can perturb the structure and orientation of the membrane-bound polypeptides. This is particularly the case for ion channel and membranelytic peptides whose amino acid composition and structural properties result in membrane-bound forms ‘poised’ between surface and membraneinserted states [2-41. In the light of these difficulties, extraction of the (normally helical) polypeptide into an ‘isotropic’ environment (a non-aqueous solution or detergent micelle) suitable for high-resolution NMR spectroscopy is often used to obtain structural and dynamic information [5]. Such studies are undertaken with the implicit assumption that the polypeptide has ‘intrinsic structural properties’ that can be assessed in the environment used to substitute for the membrane. Several questions follow that are the subject of this brief review. First, to what extent can intrinsic conformational properties really be defined and second, to what extent are they relevant for the membrane-bound polypeptide? Third, if a polypeptide has intrinsic (helical) properties, can these be predicted either by simulation or by comparison with semi-empirical methods for defining sequence-dependent helical propensities. The latter point is of interest in relation to the ‘two-stage model’ for membrane protein folding proposed by Popot and Engelman [6], in which the transmembrane helices of membrane proteind are proposed to be autonomously stable in a bilayer environment. If the limits of stable hydrogen bonding in transmembrane helices can be defined accurately, either by prediction from their sequences, by definition of ‘intrinsic’ helical