No.42 - è¾²æ¥­ç”Ÿç‰©è³‡æºç ”ç©¶æ‰€

44
Takashi HIRAYAMA and Tsutomu UGAJIN
perception. The ran1 mutants show the ethylene phenotype in response to treatment with transcyclooctene,
a potent ethylene binding inhibitor, which normally inhibits the ethylene response.
The ran1 mutants seem to have a relaxed ligand-specificity since they respond normally to
ethylene. The RAN1 gene has been identified by map based cloning and shown to encode a P-type
copper transporter similar to human Menkes or Wilson disease protein, or yeast Ccc2p. These
proteins have been demonstrated to deliver copper ions to the post Golgi compartment where
copper requiring proteins are modified and sorted subsequently to the plasma membrane or outside
of the cell, etc. These finding lead us the idea that RAN1 delivers copper ions to ethylene
receptors. Several RAN1 co-suppressed transgenic lines and the null-type ran1 mutant (ran1-3)
exhibit strong ethylene constitutive responding phenotypes (HIRAYAMA et al. 1999; WOESTE and
KIEBER 2000). In the ran1-3 mutant, ethylene receptors are expressed at the same level as wild
type (ZHAO et al. 2002). These results strongly suggest that ethylene receptors cannot function
without copper delivery.
However, this conclusion seems inconsistent with the etr1-1 phenotype. As discussed above
the conversion Cys65 to Tyr confers an ethylene insensitive phenotype, while the defect in RAN1
results in the constitutive activation of the ethylene response. One possible and plausible
explanation for this discrepancy is that Cys65 is required not only for copper coordination but also
for the proper conformation of ethylene receptor. Presumably, ethylene receptor is locked at active
state without Cys65.
Since both of the ran1-1 and ran1-2 mutations are missense mutations, these mutated genes
presumably express mutated copper transporters. These mutations cause the conversion of
important amino acid residue for copper transporting activity to another residue, indicating the
mutated copper transporters have reduced activities. Actually a recombinant ran1-1 protein has a
reduced copper transporting activity in the yeast cells (HIRAYAMA et al. 1999). If one copper ion is
incorporated in a functional ethylene receptor as Bleecker’s group proposed, the ran1-1 or ran1-2
plant would have just two types of ethylene receptors, receptor with copper or without copper.
However, having these two types of ethylene receptors cannot explain the relaxed ligand specificity
of the ran1-1 and ran1-2 mutants. Determination of the fine structure of ethylene recognition
domain is necessary.
Although all the ethylene receptors have a His-kinase like domain, the amino acid sequences are
different among them. ETR1 and ERS1 have all the motifs that are required for His-kinase activity,
namely H, N, G1, F and G2 motifs. By contrast, ETR2, EIN4 and ERS2 lack some or all of them,
indicating these His-kinase like domains do not have His-kinase activity. Recently, Wang et al.
reported that the physiological roles of His-kinase accompanied receptors, ETR1, ERS1, are different
from those of ETR2, EIN4 and ERS2, and that their His-kinase activities are not required for the
ethylene response (WANG et al. 2003). Furthermore, NTHK1, a tobacco ethylene receptor, was
shown to have Ser/Thr kinase activity (XIE et al. 2003). Based on these reports, it is more unlikely
that the ethylene-signaling pathway belongs to His-Asp phospho-relay signaling pathway.