FIGURE 3.

Transport of [³H]niacin (a) and [³H]riboflavin (b) into proteoliposomes containing EcfAA′T-NiaX and EcfAA′T-RibU, respectively. Error bars indicate the S.E. of three measurements. The proteoliposomes were loaded with 50 mm potassium phosphate, supplemented with 10 mm MgSO₄ and 10 mm ATP (closed circles), 10 mm MgSO₄ only (open circles), or 10 mm MgSO₄ and 10 mm ADP (closed triangles). The pH was 7. The accumulation levels of niacin (at the 3-min time point) and riboflavin (at the 6-min time point) are 8.7- and 26-fold, respectively. The non-zero levels of radioactivity (riboflavin or niacin) counted using the proteoliposomes loaded with Mg-ADP or MgSO₄ only are due to nonspecific binding of the label to the proteoliposomes. Similar levels were observed when proteoliposomes of an unrelated protein were used (the secondary active aspartate transporter Glt_Ph, data not shown).