Membrane transport proteins case study

The aquaporins are a family of small, integral membrane proteins that function as plasma membrane transporters of water and in some cases small polar solutes. There are at least 10 distinct aquaporins in mammals with specific expression patterns in epithelial, endothelial, and other tissues. Studies in aquaporin-null mice indicated a key role for aquaporins in the urinary concentrating mechanism, fluid secretion of glands, brain swelling, skin moisture, hearing and vision, and gastrointestinal absorption.62... 

It could well be that many of the results of increased transport rates are determined by membrane damage, rather than by reversible physical effects when surfactants are used above their CMC. Studies on the solubilization of protein have in several cases demonstrated a remarkable parallel between solubilizing ability and membrane action (30). 

The uptake of potassium by microorganisms has been well studied. In the case of E. coli, kinetic investigations on different strains have demonstrated the presence of three or four transport systems. The presence of inducible pathways with widely different Km values for binding K+ allows the cell to accumulate K+ to a constant level under different environments. These transport pathways include those linked to the proton circuit and an example linked to a pump and ATP hydrolysis. Thus the Kdp system is a high affinity pathway with Km = 1 mol dm-3, and involves three proteins in the inner membrane of E. coli, including a K+-stimulated membrane ATPase. The KHA (i.e. K+-H+ antiport) path is driven by proton motive force, while the low affinity system TrKA depends on both ATP and the proton motive force.80,81,82 S. cerevisiae accumulates K+ by K+/H+ exchange.83 Potassium transport may thus be used to control intracellular pH. 

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