Structure-function relationships of integral membrane proteins: membrane transporters vs channels

Escherichia coli lactose permease, a paradigm for membrane transport proteins, and Streptomyces lividans KcsA, a paradigm for K+ channels, are compared on the level of structure, dynamics, and function. The homotetrameric channel, which allows the downhill movement of K+ with an electrochemical gradient, is relatively rigid and inflexible, as observed by Fourier transform infrared spectroscopy. Lactose permease catalyzes transduction of free energy stored in an electrochemical H+ gradient into work in the form of a concentration gradient. In marked contrast to KcsA, the permease exhibits a high degree of H/D exchange, in addition to enhanced sensitivity to lateral lipid packing pressure, thereby indicating that this symport protein is extremely flexible and conformationally active. Finally, the differences between lactose permease and KcsA are discussed in the context of their specific functions with particular emphasis on differences between coupling in symport proteins and gating in channels.