Biology Reference
In-Depth Information
patterning, trichome development and epidermal cell differentiation. It
involves three classes of proteins, MYB, basic helix-loop-helix (bHLH) and
WD40 (named because they contain a domain of around 40 amino acids
often terminating with tryptophan and aspartic acid; reviewed in
Ramsay
and Glover, 2005
). The WD40 protein TRANSPARENT TESTAGLABRA1
(TTG1) allows different combinations of protein-protein interactions between
MYB and bHLH proteins. In the case of anthocyanin biosynthesis, a ternary
protein complex is formed involving TTG1, the bHLH proteins GLABRA3
(GL3), ENHANCER OF GLABRA3 (EGL3) or TRANSPARENT TESTA
8 (TT8) and the R2R3-MYB proteins AtMYB75 (also called PAP1),
AtMYB90 (PAP2), AtMYB113 or AtMYB114. Studies on these interactions
led to the hypothesis of a combinatorial model of gene regulation based on
these putative combinations of ternary complexes, where TTG1 is the most
general partner and the MYB proteins are the most specific partner responsible
for determining cell and function specificity (
Gonzalez et al.,2009;Zhang
et al., 2003; Zimmermann et al.,2004
).
Using the yeast two-hybrid (Y2H) system and Bi fluorescence complemen-
tation (BiFC) in planta, it was shown that PAP1 can also interact with KNAT7
(
Bhargava et al.,2010
). Therefore, the fact that PAP1 could be part of the
WD40 and bHLH ternary complexes and also interact with KNAT7 suggests
that multiple protein complexes might share some specific TFs, thereby greatly
increasing the combinatorial diversity and allowing these complexes to cross-
regulate different metabolic pathways by functioning as transcriptional acti-
vators or repressors. A further level of complexity was provided by evidence of
an interaction between KNAT7 and OVATE FAMILY PROTEINS (OFP1
and 4) using the Y2H approach and BiFC in Arabidopsis protoplasts (
Li et al.,
2011
). These three proteins act as transcriptional repressors, thus indicating
that these protein complexes may repress some aspects of SW formation (
Li
et al., 2011; Wang and Dixon, 2012
).
Another kind of protein-protein interaction regulating the phenylpropa-
noid pathway through PAP1 was discovered by studying the mediator
complex, a multiprotein complex that physically links the regulatory signals
from upstream regulatory proteins to the basal transcription machinery at
the core promoter. Using coimmunoprecipitation with antibodies against
two subunits of the mediator complex in Arabidopsis, two homolog proteins
were isolated as interactors of this complex, REDUCED EPIDERMAL
FLUORESCENCE4 (REF4) and REF4 RESEMBLING1 (RFR1;
B¨ckstr ¨m et al., 2007
). Dominant loss-of-function mutants of REF4,
inserted in the anthocyanin-accumulating pap1-D mutant, characterized
by a constitutive expression of the PAP1 gene, suppressed the anthocya-
nin-accumulating phenotype, thus suggesting that REF4 and RFR1 may
play a role in the mediator complex suppressing the expression of
