Multivalent Scaffold Proteins and Membrane Rafts:- Plants Detect Ethylene through a Two-Component System and a MAPK Cascade
The receptors for the plant hormone ethylene (CH2=CH2) are related in primary sequence to the receptor His kinases of the bacterial two-component systems and probably evolved from them; the cyanobacterial origin of chloroplasts (see Fig. 1–36) may have brought the bacterial signaling genes into the plant cell nucleus. In Arabidopsis, the two-component signaling system is contained within a single protein. The first downstream component affected by ethylene signaling is a protein Ser/Thr kinase (CTR-1; Fig. 12–29) with sequence homology to Raf, the protein kinase that begins the MAPK cascade in the mammalian response to in sulin (see the comparison in Fig. 12–30). In plants, in the absence of ethylene, the CTR-1 kinase is active and inhibits the MAPK cascade, preventing transcription of ethylene-responsive genes. Exposure to ethylene inactivates the CTR-1 kinase, thereby activating the MAPK cascade that leads to activation of the transcription fac tor EIN3. Active EIN3 stimulates the synthesis of a second transcription factor (ERF1), which in turn activates transcription of a number of ethylene-responsive genes; the gene products affect processes ranging from seedling development to fruit ripening.
Although apparently derived from the bacterial two component signaling system, the ethylene system in Arabidopsis is different in that the His kinase activity that defines component 1 in bacteria is not essential to the transduction in Arabidopsis. The genome of the cyanobacterium Anabaena encodes proteins with both an ethylene-binding domain and an active His kinase domain. It seems likely that in the course of evolution, the ethylene receptor of vascular plants was derived from that of a cyanobacterial endosymbiont, and that the bacterial His kinase became a Ser/Thr kinase in the plant.
FIGURE 12–29 Transduction mechanism for detection of ethylene by plants. The ethylene receptor in the plasma membrane (red) is a two-component system contained within a single protein, which has both a receptor domain (component 1) and a response regulator do main (component 2). The receptor controls (in ways we do not yet understand) the activity of CTR1, a protein kinase similar to MAPKKKs and therefore presumed to be part of a MAPK cascade. CTR1 is a negative regulator of the ethylene response; when CTR1 is inactive, the ethylene signal passes through the gene product EIN2 (thought to be a nuclear envelope protein), which somehow causes increased syn thesis of ERF1, a transcription factor; ERF1 in turn stimulates expression of proteins specific to the ethylene response.
