Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

882 Chapter 15: Cell Signaling
in sequence to the Raf MAP kinase kinase kinase discussed earlier (see Figure
15–49). Surprisingly, it is the empty receptors that are active and keep CTR1 active.
By an unknown signaling mechanism, active CTR1 stimulates the ubiquitylation
and degradation in proteasomes of a nuclear transcription regulator called EIN3,
which is required for the transcription of ethylene-responsive genes. In this way,
the empty but active receptors keep ethylene-response genes off. Ethylene binding
inactivates the receptors, altering their conformation so that they no longer
activate CTR1. The EIN3 protein is no longer ubiquitylated and degraded and can
now activate the transcription of the large number of ethylene-responsive genes
(Figure 15–70).
Regulated Positioning of Auxin Transporters Patterns Plant Growth
The plant hormone auxin, which is generally indole-3-acetic acid (Figure
15–71A), binds to receptor proteins in the nucleus. It helps plants grow toward
light, grow upward rather than branch out, and grow their roots downward. It also
regulates organ initiation and positioning and helps plants flower and bear fruit.
Like ethylene (and like some of the animal signal molecules we have described
in this chapter), auxin influences gene expression by controlling the degradation
of transcription regulators. It works by stimulating the ubiquitylation and degradation
of repressor proteins that block the transcription of auxin target genes in
unstimulated cells (Figure 15–71B and C).
Auxin is unique in the way that it is transported. Unlike animal hormones,
which are usually secreted by a specific endocrine organ and transported to target
(A) ABSENCE OF ETHYLENE
(B) PRESENCE OF ETHYLENE
active ethylene
receptor
copper ions
inactive
ethylene
receptor
ethylene
CYTOSOL
ER membrane
CTR1
active
kinase
domain
inactive
CTR1
polyubiquitin
chain
EIN3
degradation in
proteasomes
EIN3
ETHYLENE-RESPONSIVE
GENES OFF
TRANSCRIPTION OF ETHYLENE
TARGET GENES
Figure 15–70 The ethylene signaling pathway. (A) In the absence of ethylene, the receptors and
CTR1 are active, causing the ubiquitylation and destruction of EIN3, the transcription regulatory
protein in the nucleus that is responsible for the transcription of ethylene-responsive genes. (B) The
binding of ethylene inactivates the receptors and disrupts the activation of CTR1. The EIN3 protein
is not degraded and can therefore activate the transcription of ethylene-responsive genes.