Pore-forming subunit of a potassium efflux system that confers protection against electrophiles. Catalyzes K(+)/H(+) antiport. Interacts with the regulatory subunit KefG. Belongs to the monovalent cation:proton antiporter 2 (CPA2) transporter (TC 2.A.37) family. KefB subfamily. Could be the product of a pseudogene. Is truncated in the N-terminal where it lacks 31 residues compared to orthologs and it has a 13 residues deletion in between Ala-325 and Pro-326. Truncated N-terminus. Truncated N-terminus.