No.42 - è¾²æ¥­ç”Ÿç‰©è³‡æºç ”ç©¶æ‰€

ETHYLENE-SIGNALING PATHWAY
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signaling pathway turns on. However, as discussed below, the biochemical property of active
ethylene receptor has not been elucidated yet.
As ethylene is a tiny olefin molecule, it has been proposed that transition metals such as
copper or zinc ion are required for ethylene recognition. Bleecker’s group has demonstrated that
copper (I) ion is required for ethylene binding, and that Cys65 and His69 residues in the second
membrane spanning domain are required for copper coordination and ethylene binding activities
using recombinant ETR1 proteins expressed in the budding yeast cell (RODRIGUEZ et al. 1999).
Based on these results, they proposed a model for ethylene recognition in which two Cys65
residues and two His69 residues of ETR1 dimer coordinate with one copper (I) ion that is able to
bind one ethylene molecule (Figure 2). These results, however, did not offer any idea on the
linkage between copper requirement and ethylene receptor activity. It could be speculated at this
time that ethylene receptor is active without copper ion since the conversion Cys65 to Tyr (etr1-1
mutation) confers an ethylene insensitive phenotype.
It is the analysis of the ran1 mutants that offered in planta functional evidence for the copper
requirement for both ethylene perception and the proper conformation of ethylene receptors
(HIRAYAMA et al. 1999). The ran1 mutants were isolated in a screen for mutants with altered
ethylene-recognition specificities aiming to gain insight into the mechanism of ethylene
Figure 2 A proposed model for ethylene recognition. Two ethylene receptor
molecules constitute a functional ethylene receptor complex. Cys65
and His69 on the second membrane-spanning domain coordinate
with one copper (I) ion that binds one ethylene molecule.