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 Schizosacch aromyces pombe, that the two monosaccharide/H+ symporters HUP1 and HUP 2 of Chlorella kessleri differ significantly concerning their substrat e specificity: HUP1 transports predominantly D-glucose while HUP2 pref ers D-galactose. Several chimeric transporters were constructed and th eir substrate specificities determined. Surprisingly, it is sufficient to replace the first part of the external loop 1 of the HUP1 symporte r by the corresponding portion of HUP2 to improve transport and also t o decrease the K-m value for D-galactose, Additional data indicating t he importance of the first loop for substrate recognition and binding are discussed.