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Major intrinsic protein (MIP) superfamily. Members of the MIP superfamily function as membrane channels that selectively transport water, small neutral molecules, and ions out of and between cells. The channel proteins share a common fold: the N-terminal cytosolic portion followed by six transmembrane helices, which might have arisen through gene duplication. On the basis of sequence similarity and functional characteristics, the superfamily can be subdivided into two major groups: water-selective channels called aquaporins (AQPs) and glycerol uptake facilitators (GlpFs). AQPs are found in all three kingdoms of life, while GlpFs have been characterized only within microorganisms.
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