Membrane proteins specific lipid requirement

Opekarova, M. and Tanner, W. Specific lipid requirements of membrane proteins— a putative bottleneck in heterologous expression. Biochim Biophys Acta 1610 11— 22, 2003. 

A specific phospholipid requirement has been determined for optimum in vitro reconstitution of function for more than 50 membrane proteins. If one considers specific lipid requirements for membrane association and activation of amphitropic proteins, the number is in hundreds. Integral membrane proteins fold and exist in a very complex environment and have three modes of interaction with their environment. The extramembrane domains are exposed to the water milieu, where they interact with water, solutes, ions, and water-soluble proteins. Part of the protein is exposed to the hydrophobic-aqueous interface region (Fig. 9). The remainder of the protein is buried within the approximately 30-A thick hydrophobic interior of the membrane. Amphitropic proteins may spend part of their time completely in the cytosol and are recruited to the membrane surface, or even partially inserted into the membrane, in response to various signals. 

Therefore, if the recipient cell does not express the specific lipids required by the receptor (which may concern the acyl chain content of sphingolipids), or an adequate cholesterol-sphingolipid balance, the transfection experiment may lead to an abxmdant expression of a totally inactive receptor. In 2003, Opekarova and Tanner published a list of more than 30 membrane proteins whose activity is specifically affected by lipids. The list covered a broad range of proteins expressed by various bacteria, yeasts, insect, and mammalian cells. The problem is particularly acute when mammalian receptors or transporters are expressed in bacteria. For instance, the failure to express fxmctional serotonin transporters in E. coli has been attributed to the lack of cholesterol in bacteria. Moreover, the recovery of fully active neurotensin and adenosine receptors in transfected bacteria required the presence of choles-teryl hemisuccinate (a cholesterol derivative) during solubilization. Paradoxical results have also been obtained for some proteins whose activity requires cholesterol but can be fxmctionally expressed in bacterial hosts. In this case, one can exclude a direct interaction of cholesterol with the protein but rather consider a more general effect of the sterol on membrane properties. As a matter of fact, we are just at the beginning of our comprehension of the complex molecular ballet that involves bofh lipid and protein actors in the plasma membrane of excitable cells. 

While recent attention has been largely on proteins, it should be borne in mind that membrane fusion ultimately involves the merger of phospholipid bilayers. However, little is known about the specific membrane lipid requirements. When membranes fuse, energetically unfavorable transition states are generated that may require specific lipids and lipid domains for stabilization. Although there is some evidence for a specific influence of lipids on exocytosis, it is still unclear whether specific lipid metabolites are needed or even generated at the site of membrane merger. 

Purified membrane proteins or enzymes can be incorporated into these vesicles in order to assess what factors (eg, specific lipids or ancillary proteins) the proteins require to reconstitute their function. Investigations of purified proteins, eg, the Ca " ATPase of the sarcoplasmic reticulum, have in certain cases suggested that only a single protein and a single lipid are required to reconstitute an ion pump. 

In series with a desolvation energy barrier required to disrupt aqueous solute hydrogen bonds [14], the lipid bilayer offers a practically impermeable barrier to hydrophilic solutes. It follows that significant transepithelial transport of water-soluble molecules must be conducted paracellularly or mediated by solute translocation via specific integral membrane proteins (Fig. 6). Transcellular permeability of lipophilic solutes depends on their solubility in GI membrane lipids relative to their aqueous solubility. This lumped parameter, membrane permeability,... 

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