active motif (HCGPC), while the second domain is degenerated. The transmembrane
high-chlorophyll-fluorescence-mutant protein HCF164 is localized in the thylakoid
membranes with atypical TRX motifs (WCEVC) at the stromal as well as at the
lumen side (Motohashi and Hisabori 2006; Meyer et al. 2012). About the two
isoforms WCRKC1 and WCRKC2 not much is known; however, these proteins
contain non-canonical redox sites as their names suggest (Meyer et al. 2012).
ACHT2, ACHT4, ACHT6, CDSP32, HCF164, and WCRKC1 are mostly expressed
in green tissues (Belin et al. 2015). The genes encoding ACHT1, ACHT3, and
ACHT5 have an overall low expression in tested tissues, except for mature and
germinating pollen, in which the ACHT3 gene showed very high transcript level.
How the reduction pathways for the atypical TRX are organized is not yet sufficiently investigated. One tested member was HCF164, which was reduced by TRXm
(Motohashi and Hisabori 2006). TRXm enables a transmembrane electron transfer to
the luminal HCF164 part, so that HCF164 acts as a transducer of reducing equivalents to the inner parts of the thylakoids. Whether the FTR-dependent reduction
pathway in chloroplasts is directly interacting with atypical TRX needs to be tested.
The functional insights about atypical TRX are weak compared to the other
plastidial TRX. Members of the ACHT family have a less reducing redox midpoint
potential than typical TRX and prefer 2-Cys peroxiredoxins (PRX; see below) in
comparison to NADP
+
-MDH as redox targets in vitro (Dangoor et al. 2009). Eliyahu
et al. (2015) observed in Arabidopsis mutant studies that ACHT4 is deactivating the
key enzyme of starch synthesis AGPase through oxidation of the regulatory Cys in
the AGPase small subunit APS1 under low light conditions. CDSP32 was shown to
be involved in photooxidative stress responses by interacting with plastidial PRX
(PRXQ and 2-Cys PRX) and MSRB1 (Rey et al. 2005; Tarrago et al. 2010). HCF164
acts as reductant for luminal target proteins of the photosynthetic electron transport
chain and is essential for assembling the chloroplast cytochrome b6f complex
(Lennartz et al. 2001; Motohashi and Hisabori 2006). Future studies are needed to
identify additional biological roles of atypical TRX in plastidial processes and the
connection to their typical relatives.
2.2 NADPH-Dependent Thioredoxin Reductase C System
2.2.1 The NTRC Gene Is Exclusive of Aerobic Photosynthetic
Organisms
In heterotrophic organisms, and non-photosynthetic plant cells and compartments,
the reduction of TRX relies on NADPH with the participation of an NTR. NTR are
flavoenzymes able to transfer reducing equivalents from NADPH via FAD to a
double Cys at their active site, which reduce the disulfide at the active site of TRX.
Like TRX, NTR are widely distributed in prokaryotic and eukaryotic organisms;
however, two forms of the enzyme have evolved with different molecular and kinetic
properties (Williams et al. 2000). While the enzyme from prokaryotes is, in its native
On the Elaborate Network of Thioredoxins in Higher Plants
231
high-chlorophyll-fluorescence-mutant protein HCF164 is localized in the thylakoid
membranes with atypical TRX motifs (WCEVC) at the stromal as well as at the
lumen side (Motohashi and Hisabori 2006; Meyer et al. 2012). About the two
isoforms WCRKC1 and WCRKC2 not much is known; however, these proteins
contain non-canonical redox sites as their names suggest (Meyer et al. 2012).
ACHT2, ACHT4, ACHT6, CDSP32, HCF164, and WCRKC1 are mostly expressed
in green tissues (Belin et al. 2015). The genes encoding ACHT1, ACHT3, and
ACHT5 have an overall low expression in tested tissues, except for mature and
germinating pollen, in which the ACHT3 gene showed very high transcript level.
How the reduction pathways for the atypical TRX are organized is not yet sufficiently investigated. One tested member was HCF164, which was reduced by TRXm
(Motohashi and Hisabori 2006). TRXm enables a transmembrane electron transfer to
the luminal HCF164 part, so that HCF164 acts as a transducer of reducing equivalents to the inner parts of the thylakoids. Whether the FTR-dependent reduction
pathway in chloroplasts is directly interacting with atypical TRX needs to be tested.
The functional insights about atypical TRX are weak compared to the other
plastidial TRX. Members of the ACHT family have a less reducing redox midpoint
potential than typical TRX and prefer 2-Cys peroxiredoxins (PRX; see below) in
comparison to NADP
+
-MDH as redox targets in vitro (Dangoor et al. 2009). Eliyahu
et al. (2015) observed in Arabidopsis mutant studies that ACHT4 is deactivating the
key enzyme of starch synthesis AGPase through oxidation of the regulatory Cys in
the AGPase small subunit APS1 under low light conditions. CDSP32 was shown to
be involved in photooxidative stress responses by interacting with plastidial PRX
(PRXQ and 2-Cys PRX) and MSRB1 (Rey et al. 2005; Tarrago et al. 2010). HCF164
acts as reductant for luminal target proteins of the photosynthetic electron transport
chain and is essential for assembling the chloroplast cytochrome b6f complex
(Lennartz et al. 2001; Motohashi and Hisabori 2006). Future studies are needed to
identify additional biological roles of atypical TRX in plastidial processes and the
connection to their typical relatives.
2.2 NADPH-Dependent Thioredoxin Reductase C System
2.2.1 The NTRC Gene Is Exclusive of Aerobic Photosynthetic
Organisms
In heterotrophic organisms, and non-photosynthetic plant cells and compartments,
the reduction of TRX relies on NADPH with the participation of an NTR. NTR are
flavoenzymes able to transfer reducing equivalents from NADPH via FAD to a
double Cys at their active site, which reduce the disulfide at the active site of TRX.
Like TRX, NTR are widely distributed in prokaryotic and eukaryotic organisms;
however, two forms of the enzyme have evolved with different molecular and kinetic
properties (Williams et al. 2000). While the enzyme from prokaryotes is, in its native
On the Elaborate Network of Thioredoxins in Higher Plants
231