No.42 - è¾²æ¥­ç”Ÿç‰©è³‡æºç ”ç©¶æ‰€

46
Takashi HIRAYAMA and Tsutomu UGAJIN
enhances ethylene phenotypes. This observation suggests the existence of other CTR1 like
components. There are several CTR1 like genes on the Arabidopsis genome. Although the
physiological functions of those are not known yet, these kinases might function in the ethylenesignaling
pathway.
Recently, Larsen and Chang reported another ethylene related mutant, eer1. This mutant
shows enhanced ethylene response phenotypes (LARSEN and CHANG 2001). The ctr1 eer1 double
mutant exhibits stronger constitutive ethylene response phenotypes. The EER1 gene has been
cloned and shown to be identical to RCN1 that encodes an A subunit of protein phosphatase 2A
(PP2A) (LARSEN and CANCEL 2003). RCN1 has been reported to be involved in the responses to
auxin and abscisic acid in root and in guard cell, respectively. In mammal cells, PP2A positively
regulates Raf-1 and shown to interact with Raf-1 kinase directly (ABRAHAM et al. 2000). Actually,
not RCN1 but a C subunit of PP2A, PP2A-1C, can interact with the N-terminal domain of CTR1 in
vitro. It might be possible that RCN1/EER1 and/or other PP2As are involved in the regulation of
CTR1 activity.
The ein2 mutants have a semi-dominant strong ethylene insensitive phenotype, suggesting its
important function in the ethylene-signaling pathway. However, little is known about EIN2. The
EIN2 gene encodes a novel membrane-spanning protein (ALONSO et al. 1999). EIN2 has twelve
putative membrane-spanning domains in its N-terminal half. This region has a significant
similarity to Nramp divalent cation transporters. However, there is no evidence for the transporter
activity of EIN2. In addition, two residues that have been shown to be required for the transporting
activity of yeast Smf1p are not conserved in EIN2, suggesting that EIN2 does not have such an iontransporting
activity. Although EIN2 must localize to membrane structures because of these
membrane-spanning domains, the subcellular localization of EIN2 has not been determined yet.
The C-terminal half seems to be a cytoplasmic domain. The function of this region is also not
clarified yet since this region does not have any known motifs. However, overexpression of the C-
terminal half confers constitutive ethylene response phenotypes in the absence of ethylene,
suggesting that this region has a pivotal role in the activation of the down stream signal transducer.
The mutation sites of dozens of the ein2 mutants have been determined. All of them except one
(ein2-9) are non-sense mutations or frame-sift mutations. Given that the C-terminal domain is
required for the EIN2 function, these mutant EIN2 proteins cannot activate the downstream
component(s) and result in the ethylene insensitive phenotype. ein2-9 is a missense mutation that
causes an amino acid conversion His1143 to Pro in the C-terminal domain. Since this residue is
not conserved in the rice EIN2 homologue, it is postulated that this His residue does not have a
specific function but the conversion from this His residue to Pro might disturb the functional
structure of the C-terminal region.
The relationship between EIN2 and Nramp seems similar to that of yeast glucose transporters
(HXTs) and glucose sensors (Snf3p and Rgt2p). Snf3p and Rgt2p have a structure similar to
HXTs at their N-terminal regions and a unique C-terminal cytoplasmic domain. At the beginning,