ETHYLENE RESPONSE 1 histidine kinase activity of Arabidopsis promotes plant growth

ETHYLENE RESPONSE 1 (ETR1) is one of five receptors for the plant hormone ethylene (Bleecker et al., 1988; Chang et al., 1993; for review, see Bleecker and Kende, 2000; Schaller and Kieber, 2002). ETR1 contains both an N-terminal ligand-binding hydrophobic domain and light signaling-implicated GAF domain (Fig. 1A; Chang et al., 1993). ETR1 also has a C-terminal His kinase (HK) domain fused with a response regulator motif. Biochemical analyses have demonstrated functional HK activity (Gamble et al., 1998, 2002; Moussatche and Klee, 2004). Within the HK domain, both the catalytic G1 and ATP-binding G2 domains are required for the autophosphorylation of ETR1 at His-353. Protein-protein interaction between ETR1 and any of three Arabidopsis (Arabidopsis thaliana) His phosphotransfer proteins suggests that ETR1 HK activity may modulate Arabidopsis response regulators (ARRs) through two-component phosphor-relay system (Urao et al., 2000). Indeed, ETR1-dependent phosphorylation of the B-type ARR2 activates ethylene response transcription (Hass et al., 2004). Nevertheless, signaling through ARRs modulates ethylene signaling marginally, because combinations of B-type arr mutants are still sensitive to ethylene (Mason et al., 2005). Furthermore, ETR1 with inactive HK provides wild-type ethylene responsiveness in single etr1 or double etr1 ers1 null mutants (Gamble et al., 2002; Wang et al., 2003). Thus far, the biological function or an in vivo signaling mechanism for ETR1 HK activity remains unknown. Only recently, the HK activity was shown to be important in the rate of growth recovery after ethylene removal (Binder et al., 2004). Otherwise, its role in ethylene or any other plant signaling is unclear.