Functions as two-component phosphorelay mediators between cytokinin sensor histidine kinases and response regulators (B-type ARRs). Plays an important role in propagating cytokinin signal transduction. Interacts with AHK5. A number of isoforms are produced. According to EST sequences. Lacks the conserved active histidine at position 83 that mediates the phosphotransfer. Shows a conserved HPt domain that may have some alternative degenerated phosphorelay role in cell signaling.