Potassium/proton antiporter that mediates the efflux of potassium ions from the cell (PubMed:17679694). Can also mediate rubidium/proton antiport, but has no permeability for sodium or lithium ions. In the absence of KhtT, does not have antiport activity, but can catalyze potassium efflux (PubMed:14987767, PubMed:17679694). Involved in protection of the cell from methylglyoxal, a toxic by-product of glycolysis, via activation by S-lactoyl-BSH of the antiporter activity, leading to cytoplasmic acidification and methylglyoxal resistance (PubMed:24330391). Potassium antiport activity requires the presence of KhtT. Activity is also modulated by KhtS. Has higher activity at alkaline pH. The transporter is composed of the integral membrane protein KhtU and the regulatory protein KhtT. Part of the khtSTU operon. Induced by salt stress at alkaline pH. Deletion mutant is more sensitive to methylglyoxal than wild-type. Belongs to the monovalent cation:proton antiporter 2 (CPA2) transporter (TC 2.A.37) family.