Advances in ethylene signalling: protein complexes at the endoplasmic reticulum membrane

Abstract

The gaseous plant hormone ethylene plays critical roles in plant responses to environmental and endogenous signals that modulate growth and development. Over the past 25 years, great progress has been made in elucidating the ethylene signalling pathway. Genetic studies in Arabidopsis thaliana have identified key components of the pathway, and subcellular localization studies have shown that most of these components, other than transcription factors and protein turnover machinery, are associated with or lie within the endoplasmic reticulum (ER) membrane. The ethylene receptors are found in high-molecular-mass protein complexes and interact with the CTR1 serine/threonine protein kinase and the genetically downstream EIN2 Nramp-like protein. To more fully understand the ethylene signalling pathway, recent research has focused on examining the molecular connections between these components and how they are regulated. Here, we review recent advances and remaining gaps in our understanding of the early steps in the ethylene signalling pathway taking place at the ER.

Fig. 1

A model for ethylene signalling at the ER membrane in Arabidopsis. The initial steps in the ethylene signalling pathway occur at the ER membrane and involve ethylene receptors (represented here by ETR1; Chang et al. 1993) interacting with the CTR1 serine/threonine protein kinase (Kieber et al. 1993) and the EIN2 Nramp homologue (Alonso et al. 1999). Left: the N-terminal regulatory domain of the CTR1 protein kinase associates with the ethylene receptor histidine kinase (HK) and receiver (R) domains. In the absence of ethylene, the ethylene receptors activate the CTR1 kinase domain (KD) by an unknown mechanism. Active CTR1 somehow represses EIN2. Right: ethylene binding shuts off receptor signalling, such that the CTR1 kinase domain is no longer active, allowing signalling to proceed to EIN2. We postulate that dimerization/monomerization of CTR1 could play a role in activating/inactivating the CTR1 KD, respectively. The biochemical functions of EIN2 have yet to be determined, but downstream of EIN2 there is activation of the nuclear transcription factors EIN3/EIL1 and ERF1, which induce ethylene-responsive gene expression. Interestingly, EIN2 can also associate with ethylene receptors (Bisson et al. 2009; Bisson and Groth 2010), and in vitro association of EIN2 with the ETR1 receptor is enhanced when ETR1 histidine kinase activity is disrupted (Bisson and Groth 2010). The P-type ATPase copper transporter, RAN1, provides the copper cofactor required for ethylene binding (Rodriguez et al. 1999), and is important for the biogenesis of the receptors (Hirayama et al. 1999; Woeste and Kieber 2000; Binder et al. 2010), while RTE1 activates the ETR1 receptor by an unknown mechanism (Resnick et al. 2006, 2008). The RTE1 membrane topology is unknown and is speculated here.

Fig. 2

Model of an active heteromeric ethylene receptor–CTR1 complex at the ER membrane. The ethylene receptors are tethered to the ER membrane (Chen et al. 2002, 2007; Dong et al. 2008; Grefen et al. 2008) by the N-terminal ethylene-binding domain (EBD). Representative ethylene receptors of subfamily I (in dark blue) and subfamily II (in light blue) are homodimers (Schaller and Bleecker 1995; Gao et al. 2008), each stabilized by a pair of intermolecular N-terminal disulfide bonds (S–S) in the lumen (Schaller et al. 1995), as well as likely non-covalent interactions (red arrows, shown only on the leftmost homodimer) between the two-component histidine kinase domains (HK), receiver domains and GAF domains. The receptor homodimers form a higher-order complex with neighbouring receptor dimers, mediated in part by the GAF domain (black arrows) (Schaller et al. 1995; Hall et al. 2000; Gao et al. 2008; Grefen et al. 2008). The N-terminal regulatory domain of the CTR1 protein kinase (green) physically associates with the HK and receiver domains of the receptors (Clark et al. 1998; Cancel and Larsen 2002; Gao et al. 2003; Zhong et al. 2008). We speculate that each receptor HK domain associates with one CTR1 molecule. Based on crystal structures, the ETR1 receiver domain (Müller-Dieckmann et al. 1999) and the CTR1 kinase domain (Mayerhofer et al. 2012) are each dimers (red arrows). The CTR1 kinase domain is believed to be active when dimerized (Mayerhofer et al. 2012). Moreover, oligomerization of the CTR1 kinase domain dimers (black arrows) may help to bring the ethylene receptors together, reinforcing the receptor complex (Mayerhofer et al. 2012). The receptors have also been found in high-molecular-mass complexes containing unidentified proteins (Chen et al. 2010) (not shown). The higher-order ethylene receptor complexes may serve to amplify the signal.