The folding of K K-helical membrane proteins has previously been described using the two stage model, in which the membrane insertion of independently stable K K-helices is followed by their mutual interactions within the membrane to give higher order folding and oligomerization. Given recent advances in our understanding of membrane protein structure it has become apparent that in some cases the model may not fully represent the folding process. Here we present a three stage model which gives considerations to ligand binding, folding of extramembranous loops, insertion of peripheral domains and the formation of quaternary structure. ß

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In this work we present data from a homologous series of di-pyrenyl phosphatidylcholine (dipyPC) probes which can sense lateral pressure variations in the chain region of the amphiphilic membrane (lateral pressures are tangential to the interface). The dipyPC has pyrene moieties attached to the ends of equal length acyl chains on a phosphatidylcholine molecule. Ultraviolet stimulation produces both monomer and excimer Ñuorescence from pyrene. At low dilutions of dipyPC in model membranes the excimer signal is entirely intra-molecular and since it depends on the frequency with which the pyrene moieties are brought into close proximity, the relative intensity of the excimer to monomer signal, g, is a measure of the pressure. We synthesised or purchased dipyPC probes with the pyrene moieties attached to acyl chains having 4, 6, 8 and 10 carbon atoms and then measured g in fully hydrated bilayers composed of dioleoylphosphatidylcholine and dioleoylphosphatidylethanolamine (DOPC and DOPE respectively). Although the resolution of our measurements of lateral pressure as a function of distance into the monolayer was limited, we did observe a dip in the excimer signal in the region of the DOPC/DOPE cis double bond. As we isothermally increased the DOPE composition, and hence the desire for interfacial curvature, we observed, as expected, that the net excimer signal increased. However this net increase was apparently brought about by a transfer of pressure from the region around the glycerol backbone to the region near the chain ends, with the lateral pressure dropping above the cis double bond but increasing at a greater rate beyond the double bond.

Three different lipid systems have been developed to investigate the effect of physicochemical forces within the lipid bilayer on the folding of the integral membrane protein bacteriorhodopsin. Each system consists of lipid vesicles containing two lipid species, one with phosphatidylcholine and the other with phosphatidylethanolamine headgroups, but the same hydrocarbon chains: either L-alpha-1, 2-dioleoyl, L-alpha-1,2-dipalmitoleoyl, or L-alpha-1,2-dimyristoyl. Increasing the mole fraction of the phosphatidylethanolamine lipid increases the desire of each monolayer leaflet in the bilayer to curve toward water. This increases the torque tension of such monolayers, when they are constrained to remain flat in the vesicle bilayer. Consequently, the lateral pressure in the hydrocarbon chain region increases, and we have used excimer fluorescence from pyrene-labeled phosphatidylcholine lipids to probe these pressure changes. We show that bacteriorhodopsin regenerates to about 95% yield in vesicles of 100% phosphatidylcholine. The regeneration yield decreases as the mole fraction of the corresponding phosphatidylethanolamine component is increased. The decrease in yield correlates with the increase in lateral pressure which the lipid chains exert on the refolding protein. We suggest that the increase in lipid chain pressure either hinders insertion of the denatured state of bacterioopsin into the bilayer or slows a folding step within the bilayer, to the extent that an intermediate involved in bacteriorhodopsin regeneration is effectively trapped.

Sequence motifs are responsible for ensuring the proper assembly of transmembrane (TM) helices in the lipid bilayer. To understand the mechanism by which the affinity of a common TM-TM interactive motif is controlled at the sequence level, we compared two well characterized GXXXG motif-containing homodimers, those formed by human erythrocyte protein glycophorin A (GpA, high-affinity dimer) and those formed by bacteriophage M13 major coat protein (MCP, low affinity dimer). In both constructs, the GXXXG motif is necessary for TM-TM association. Although the remaining interfacial residues (underlined) in GpA (LIXXGVXXG-VXXT) differ from those in MCP (VVXXGAXXGIXXF), molecular modeling performed here indicated that GpA and MCP dimers possess the same overall fold. Thus, we could introduce GpA interfacial residues, alone and in combination, into the MCP sequence to help decrypt the determinants of dimer affinity. Using both in vivo TOXCAT assays and SDS-PAGE gel migration rates of synthetic peptides derived from TM regions of the proteins, we found that the most distal interfacial sites, 12 residues apart (and ϳ18 Å in structural space), work in concert to control TM-TM affinity synergistically.After their biosynthesis and subsequent integration into a membrane, many transmembrane (TM) 1 helices associate with other pre-formed helices to form functional membrane protein domains (1). Specificity for a given helix-helix interaction is achieved through the appropriate presentation of complementary side chains, which serve as recognition elements between associating helices. The most highly studied, and apparently widespread, mode of association is mediated by the so-called GXXXG motif, which is known to act as a universal scaffold for the assembly of both TM helices (2-9) and soluble ␣-helices (10). The GXXXG motif, where two glycine residues are separated by any three amino acids on a helical framework, gives rise to a flat surface region on one face of the helix. This arrangement of Gly residues permits the close approach of interacting helices, whereupon extensive packing interactions take place between pairs of surrounding residues. It has been proposed that a portion of the interactive strength of GXXXGmediated associations may originate from inter-helix hydrogen bonds between C␣ hydrogens and carbonyl oxygen atoms on the adjacent helix (11).The glycophorin A transmembrane (GpA-TM) segment is a well characterized transmembrane helix dimer that associates with high affinity, principally by using a central GXXXG motif (3,12). The details of side chain-side chain packing for GpA are known in considerable detail, having been gleaned originally from extensive mutagenesis experiments (3), computer modeling (13, 14) and, subsequently, from a high-resolution structural analysis using nuclear magnetic resonance (NMR) for the GpA dimer in both detergent micelles (12) and lipid bilayers (15).Despite the high occurrence of the GXXXG motif in transmembrane helices (7), GpA-TM is the only GXXXG peptide dimer with a structure det...

The regeneration kinetics of the integral membrane protein bacteriorhodopsin have been investigated in a lipid-based refolding system. Previous studies on bacteriorhodopsin regeneration have involved detergent-based systems, and in particular mixed dimyristoylphosphatidylcholine (DMPC)/CHAPS micelles. Here, we show that the short chain lipid dihexanoylphosphatidylcholine (DHPC) can be substituted for the detergent CHAPS and that bacteriorhodopsin can be regenerated to high yield in mixed DMPC/DHPC micelles. Bacteriorhodopsin refolding kinetics are measured in the mixed DMPC/DHPC micelles. Rapid, stopped flow mixing is employed to initiate refolding of denatured bacterioopsin in SDS micelles with mixed DMPC/DHPC micelles and time-resolved fluorescence spectroscopy to follow changes in protein fluorescence during folding. Essentially identical refolding kinetics are observed for mixed DMPC/CHAPS and mixed DMPC/DHPC micelles. Only one second-order retinal/apoprotein reaction is identified, in which retinal binds to a partially folded apoprotein intermediate, and the free energy of this retinal binding reaction is found to be the same in both types of mixed micelles. Formation of the partially folded apoproptein intermediate is a rate-limiting step in protein folding and appears to be biexponential. Both apparent rate constants are found to be dependent on the relative proportion of DMPC present in the mixed DMPC/DHPC micelles as well as on the pH of the aqueous phase. Increasing the DMPC concentration should increase the bending rigidity of the amphiphilic bilayer, and this is found to slow the rate of formation of the partially folded apoprotein intermediate. Increasing the mole fraction of DMPC from 0.3 to 0.6 slows the two apparent rate constants associated with formation of this intermediate from 0.29 and 0.031 to 0.11 and 0.013 s -1, respectively. Formation of the intermediate also slows with increasing pH, from 0.11 and 0.013 s-1 at pH 6 to 0.033 and 0.0053 s-1 at pH 8. Since this pH change has no known effect on the phase behavior of lecithins, this is more likely to represent a direct effect on the protein itself. Thus, it appears to be possible to control the rate-limiting process in bacterioopsin folding through both bilayer bending rigidity and pH.

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The study of membrane protein folding is a new and challenging research field. Consequently, there are few direct studies on the in vitro folding of membrane proteins. This review covers work aimed at understanding folding mechanisms and the intermolecular forces that drive the folding of integral membrane proteins. We discuss the kinetic and thermodynamic studies that have been undertaken. Our review also draws on closely related research, mainly from purification studies of functional membrane proteins, and gives an overview of some of the successful methods. A brief survey is also given of the large body of mutagenesis and fragment work on membrane proteins, as this too has relevance to the folding problem. It is noticeable that the choice of solubilizing detergents and lipids can determine the success of the method, and indeed it appears that particular lipid properties can be used to control the rate and efficiency of folding. This has important ramifications for much in vitro folding work in that it aids our understanding of how to obtain and handle folded, functional protein. With this in mind, we also cover some relevant properties of model, lipid-bilayer systems.