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A Novel Plant Major Intrinsic Protein in Physcomitrella patens Most Similar to Bacterial Glycerol Channels

Gustavsson, Sofia, Lebrun, Anne-Sophie, Nordén, Kristina, Chaumont, François, Johanson, Urban
Plant physiology 2005 v.139 no.1 pp. 287-295
Funariaceae, mosses and liverworts, glycerol, transporters, complementary DNA, sequence analysis, recombinant fusion proteins, Xenopus laevis, oocytes, amino acid sequences, Physcomitrella patens
A gene encoding a novel fifth type of major intrinsic protein (MIP) in plants has been identified in the moss Physcomitrella patens. Phylogenetic analyses show that this protein, GlpF-like intrinsic protein (GIP1;1), is closely related to a subclass of glycerol transporters in bacteria that in addition to glycerol are highly permeable to water. A likely explanation of the occurrence of this bacterial-like MIP in P. patens is horizontal gene transfer. The expressed P. patens GIP1;1 gene contains five introns and encodes a unique C-loop extension of approximately 110 amino acid residues that has no obvious similarity with any other known protein. Based on alignments and structural comparisons with other MIPs, GIP1;1 is suggested to have retained the permeability for glycerol but not for water. Studies on heterologously expressed GIP1;1 in Xenopus laevis oocytes confirm the predicted substrate specificity. Interestingly, proteins of one of the plant-specific subgroups of MIPs, the NOD26-like intrinsic proteins, are also facilitating the transport of glycerol and have previously been suggested to have evolved from a horizontally transferred bacterial gene. Further studies on localization and searches for GIP1;1 homologs in other plants will clarify the function and significance of this new plant MIP.