When overexpressed in human HEK-293 cells forms an inward rectifying potassium channel. When overexpressed using vectors that provide a promoter and ribosome binding site (PubMed:15919996). YdfJ may constitute the C-terminal portion of an ancestral gene disrupted by an insertion of the cryptic prophage Qin/Kim. The N-terminal fragment may be ynfP. Belongs to the major facilitator superfamily. Metabolite:H+ Symporter (MHS) family (TC 2.A.1.6) family.