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enables the conversion of OAS to cysteine by the free enzyme, and in a negative
feedback, reduces the rate of OAS formation (Hesse et al.
2004b
). Therefore,
formation of the cysteine synthase complex appears to be a main regulatory step
in cysteine synthesis (see the “Regulation of cysteine synthesis - protein-protein
interactions” section in this chapter).
The crystallisation of SAT protein revealed that it is a hexamer composed from
29 kDa subunits which are folded in a left handed parallel
ʲ
-helix, characteristic for
this protein family (Olsen et al.
2004
). The SAT gene family includes five members
in
Arabidopsis
, two of which were recognised only recently (Hell and Wirtz
2008
;
Kawashima et al.
2005
). Among the five SAT proteins three of them, SAT2, 4, and
5 (Serat 3.1, 3.2, and 1.1, respectively) are located in the cytosol whereas SAT1
(Serat 2.1) was found in plastids and SAT3 (Serat 2.2) in mitochondria. Addition-
ally, because of the very low substrate affinity of SAT2 and 4 compared to SAT1,
3, and 5, it was suggested that these isoenzymes may actually process different
substrates
in vivo
(Kawashima et al.
2005
; Krueger et al.
2009
). However, the
analysis of multiple knock-out mutants for different SAT isoforms revealed that all
of
Arabidopsis
SAT are able to complement, at least partially, for the loss of other
isoforms (Watanabe et al.
2008
).
OAS-TL belongs to the
-replacement enzyme family which requires pyridoxal-
5
0
-phosphate as a cofactor. The protein is a homodimer composed from 35 kDa
subunits. The
Arabidopsis
gene family contains nine members which encode eight
functionally transcribed proteins (Hell and Wirtz
2008
). Three of them, called
OAS-TL A, B, and C, are thought to be the main OAS-TL proteins in plant cells.
Similarly to SAT, they are localised in the cytosol, plastids and mitochondria,
respectively (Wirtz et al.
2004
). OAS-TL proteins seem to have wide range of
functions. It is likely that apart from cysteine synthesis they are also responsible for
other processes such as sulfide and cyanide detoxification in mitochondria (Alvarez
et al.
2012
) or determination of antioxidative capacity in cytosol (L
´
pez-Mart
´
n
et al.
2008
). They also seem to be involved in the synthesis of secondary metabo-
lites in various species.
Recent studies of SAT and OAS-TL mutants suggest the cytosol as the main cell
compartment for cysteine production and mitochondria as the main place for OAS
synthesis (Haas et al.
2008
; Krueger et al.
2009
; Watanabe et al.
2008
). Plants with
decreased mitochondrial SAT activity show strongly reduced OAS levels and
reduced flux into cysteine and GHS (Haas et al.
2008
). The analyses of OAS
content and SAT activity in non-aqueous gradients showed the largest amount of
OAS in mitochondria and the smallest in plastids, whereas the OAS-TL activity was
localised in the cytosol and plastids (Krueger et al.
2009
). However, OAS can be
transferred between all three compartments. Additionally, the analysis of
compartment-specific OAS-TL mutants revealed reduced cysteine content only in
a mutant lacking the cytosolic OAS-TL isoform (Haas et al.
2008
; Watanabe
et al.
2008
). Consequently, OAS has to be transported from mitochondria to the
cytosol for efficient cysteine biosynthesis. Taking into account that plastids are the
main compartment for sulfide production, the presence of sulfide in cytosol also
requires sulfide transport across the chloroplast envelope membrane. Taken
ʲ
