Nucleotide sequence and 3'-end deletion studies indicate that the K(+)-uptake protein kup from Escherichia coli is composed of a hydrophobic core linked to a large and partially essential hydrophilic C terminus.

The kup (formerly trkD) gene from Escherichia coli encodes a minor K(+)-uptake system. The gene is located just upstream of the rbsDACBK operon at 84.5 min on the chromosome and is transcribed clockwise. kup codes for a 69-kDa protein, which may be composed of two domains. The first 440 amino acid residues appear to form an integral membrane protein that might traverse the cell membrane 12 times. The C-terminal 182 amino acid residues are predicted to form a hydrophilic domain located at the cytoplasmic side of the membrane. Deletion studies from the 3' end of kup showed that removal of almost the complete hydrophilic domain of the protein reduced, but did not abolish, K(+)-uptake activity.