BACTERIAL SOLUTE TRANSPORT PROTEINS IN THEIR LIPID ENVIRONMENT

The cytoplasmic membrane of bacteria is a selective barrier that restr icts entry and exit of solutes. Transport of solutes across this membr ane is catalyzed by specific membrane proteins. Integral membrane prot eins usually require specific lipids for optimal activity and are inhi bited by other lipid species. Their activities are also sensitive to t he lipid bilayer dynamics and physico-chemical state. Bacteria can ada pt to changes in the environments (respective temperature, hydrostatic pressure, and pH) by altering the lipid composition of the membrane. Homeoviscous adaptation results in the maintenance of the liquid-cryst alline phase through alterations in the degree of acyl chain saturatio n and branching, acyl chain length and the sterol content of the membr ane. Homeophasic adaptation prevents the formation of non-bilayer phas es, which would disrupt membrane organization and increase permeabilit y. A balance is maintained between the lamellar phase, preferring lipi ds, and those that adopt a non-bilayer organization. As a result, the membrane proteins are optimally active under physiological conditions. The molecular basis of lipid-protein interactions is still obscure. A nnular lipids stabilize integral membrane proteins. Stabilization occu rs through electrostatic and possibly other interactions between the l ipid headgroups and the charged amino acid residues close to the phosp holipid-water interface, and hydrophobic interactions between the fatt y acyl chains and the membrane-spanning segments. Reconstitution techn iques allow manipulation of the lipid composition of the membrane in a way that is difficult to achieve in vivo. The physical characteristic s of membrane lipids that affect protein-mediated transport functions have been studied in liposomal systems that separate an inner and oute r compartment. The activity of most transport proteins is modulated by the bulk physical characteristics of the lipid bilayer, while specifi c lipid requirements appear rare.