from the interaction of the two subunits with each other (Tarun and Theologis 1998;
Tsuchisaka and Theologis 2004a). In Arabidopsis, there exist 12 genes encoding
ACS but among them only eight genes (ACS2, 4–9, and 11) encode functional ACS
(Bleecker and Kende 2000; Yamagami et al. 2003). Indeed, complementation of the
E. coli aminotransferase mutant DL39 showed that ACS10 and ACS12 are aminotransferases with broad specificity for aspartate and aromatic amino acids whereas
ACS1 forms inactive homodimers and ACS3 was a pseudogene (Tsuchisaka and
Theologis 2004a). Based on protein sequences comparison, three subtypes of the
eight functional ACS have been defined mainly based on the number of phosphorylation sites in their C-terminal sequence (Chae and Kieber 2005). These sites are
involved in posttranslational regulation and modulate the stability and degradation
of ACS proteins (Wang et al. 2004; Argueso et al. 2007; Hansen et al. 2009). Thus,
the phosphorylation of ACS-type 1 (ACS1, ACS2, ACS6) are controlled by mitogen
activated protein kinase (Tatsuki and Mori 2001; Liu and Zhang 2004) while the
phosphorylation of ACS-type 2 (ACS4, ACS5, ACS8, ACS9, ACS11) are regulated
by casein kinase (Tan and Xue 2014). The ACS-type 3 (ACS7) is a more stable
protein because it has no C-terminal phosphorylation site but ubiquitin regulated its
degradation via proteasome (Lyzenga et al. 2012).
Although ACS genes are strongly regulated at the transcriptional level depending
on many developmental and environmental factors (Van der Straeten et al. 1992;
Tsuchisaka and Theologis 2004b), to our knowledge only a few studies have compared
the promoter sequences of ACS genes and search for common or specific responsive
elements to transcription factors (Rodrigues-Pousada et al. 1993; Wand et al. 2005).
As with many aminotransferases, it is likely that transcription of ACS genes is also
controlled by more general mechanisms that affect the nutritional and energy status of
the cell such as GCN2 and TOR kinase signaling systems (Xiong and Sheen 2015).
Briefly, in plants GCN2 (General Control Non-derepressible 2) kinase is a cellular
sensing system that allows overcoming amino acid starvation and stress conditions
during growth (Hey et al. 2010; Zhang et al. 2003; Lageix et al. 2008; Castilho et al.
2014), whereas TOR (Target Of Rapamycin) kinase is a key regulator conserved in
eukaryotic cells that controls nutrients and energy signaling topromote cell proliferation
and growth (Loewith and Hall 2011; Xiong and Sheen 2014).
4.3 Cytosolic Levels of ACC Can Be Modulated
in Many Ways
ACC produced by activity of the ACS isoforms is a neutral and non-proteinogenic
amino acid that displays a structural analogy and molecular mass close to other
signaling amino acids molecules such as GABA, α- and β-aminobutyric acids
(α-ABA and β-ABA). If ACS is a rate-limiting step in ACC biosynthesis (Fig. 4),
there are at least four different ways to modulate the excessive levels of ACC
produced by ACS activation.
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
267
Tsuchisaka and Theologis 2004a). In Arabidopsis, there exist 12 genes encoding
ACS but among them only eight genes (ACS2, 4–9, and 11) encode functional ACS
(Bleecker and Kende 2000; Yamagami et al. 2003). Indeed, complementation of the
E. coli aminotransferase mutant DL39 showed that ACS10 and ACS12 are aminotransferases with broad specificity for aspartate and aromatic amino acids whereas
ACS1 forms inactive homodimers and ACS3 was a pseudogene (Tsuchisaka and
Theologis 2004a). Based on protein sequences comparison, three subtypes of the
eight functional ACS have been defined mainly based on the number of phosphorylation sites in their C-terminal sequence (Chae and Kieber 2005). These sites are
involved in posttranslational regulation and modulate the stability and degradation
of ACS proteins (Wang et al. 2004; Argueso et al. 2007; Hansen et al. 2009). Thus,
the phosphorylation of ACS-type 1 (ACS1, ACS2, ACS6) are controlled by mitogen
activated protein kinase (Tatsuki and Mori 2001; Liu and Zhang 2004) while the
phosphorylation of ACS-type 2 (ACS4, ACS5, ACS8, ACS9, ACS11) are regulated
by casein kinase (Tan and Xue 2014). The ACS-type 3 (ACS7) is a more stable
protein because it has no C-terminal phosphorylation site but ubiquitin regulated its
degradation via proteasome (Lyzenga et al. 2012).
Although ACS genes are strongly regulated at the transcriptional level depending
on many developmental and environmental factors (Van der Straeten et al. 1992;
Tsuchisaka and Theologis 2004b), to our knowledge only a few studies have compared
the promoter sequences of ACS genes and search for common or specific responsive
elements to transcription factors (Rodrigues-Pousada et al. 1993; Wand et al. 2005).
As with many aminotransferases, it is likely that transcription of ACS genes is also
controlled by more general mechanisms that affect the nutritional and energy status of
the cell such as GCN2 and TOR kinase signaling systems (Xiong and Sheen 2015).
Briefly, in plants GCN2 (General Control Non-derepressible 2) kinase is a cellular
sensing system that allows overcoming amino acid starvation and stress conditions
during growth (Hey et al. 2010; Zhang et al. 2003; Lageix et al. 2008; Castilho et al.
2014), whereas TOR (Target Of Rapamycin) kinase is a key regulator conserved in
eukaryotic cells that controls nutrients and energy signaling topromote cell proliferation
and growth (Loewith and Hall 2011; Xiong and Sheen 2014).
4.3 Cytosolic Levels of ACC Can Be Modulated
in Many Ways
ACC produced by activity of the ACS isoforms is a neutral and non-proteinogenic
amino acid that displays a structural analogy and molecular mass close to other
signaling amino acids molecules such as GABA, α- and β-aminobutyric acids
(α-ABA and β-ABA). If ACS is a rate-limiting step in ACC biosynthesis (Fig. 4),
there are at least four different ways to modulate the excessive levels of ACC
produced by ACS activation.
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
267