Importance of the first external loop for substrate recognition as revealed by chimeric Chlorella monosaccharide/H+ symporters.
It had been shown previously by heterologous expression in Schizosaccharomyces pombe, that the two monosaccharide/H+ symporters HUP1 and HUP2 of Chlorella kessleri differ significantly concerning their substrate specificity: HUP1 transports predominantly D-glucose while HUP2 prefers D-galactose. Several chimeric transporters were constructed and their substrate specificities determined. Surprisingly, it is sufficient to replace the first part of the external loop 1 of the HUP1 symporter by the corresponding portion of HUP2 to improve transport and also to decrease the Km value for D-galactose. Additional data indicating the importance of the first loop for substrate recognition and binding are discussed.
Will
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Lehrstuhl für Zellbiologie und Pflanzenphysiologie, Universität Regensburg, Germany.
Tanner
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D000595:Amino Acid Sequence; D002352:Carrier Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D002708:Chlorella / Q000378:metabolism*; D003001:Cloning, Molecular; D018076:DNA, Complementary; D004926:Escherichia coli; D007700:Kinetics; D008565:Membrane Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D008961:Models, Structural; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D017433:Protein Structure, Secondary; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D027981:Symporters
D002352:Carrier Proteins; D018076:DNA, Complementary; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D011993:Recombinant Fusion Proteins; D027981:Symporters; C089820:monosaccharide-hydrogen cotransporter