Glycoside Hydrolase Family 9/Plant endoglucanases - Revision history

Harry Brumer: Text replacement - "\^\^\^(.*)\^\^\^" to "$1"

2021-12-18T21:18:23Z

Text replacement - "\^\^\^(.*)\^\^\^" to "$1"

Breeanna Urbanowicz at 20:12, 9 March 2015

2015-03-09T20:12:53Z

Breeanna Urbanowicz at 19:42, 9 March 2015

2015-03-09T19:42:35Z

Breeanna Urbanowicz at 19:41, 9 March 2015

2015-03-09T19:41:24Z

Breeanna Urbanowicz at 19:39, 9 March 2015

2015-03-09T19:39:36Z

Breeanna Urbanowicz at 18:17, 9 March 2015

2015-03-09T18:17:49Z

Breeanna Urbanowicz at 17:55, 9 March 2015

2015-03-09T17:55:03Z

Breeanna Urbanowicz at 14:34, 30 August 2010

2010-08-30T14:34:53Z

Spencer Williams at 06:31, 29 August 2010

2010-08-29T06:31:24Z

Spencer Williams at 04:27, 29 August 2010

2010-08-29T04:27:22Z

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 {{CuratorApproved}}
-* [[Author]]: ^^^Breeanna Urbanowicz^^^
+* [[Author]]: [[User:Breeanna Urbanowicz|Breeanna Urbanowicz]]
-* [[Responsible Curator]]:  ^^^David Wilson^^^
+* [[Responsible Curator]]:  [[User:David Wilson|David Wilson]]
 ----

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 The Class A EGases are integral type II membrane proteins with a [[GH9]] catalytic core that lack a canonical secretion signal sequence. These enzymes are predicted to have a high degree of N-glycosylation and a long amino-terminal extension with a membrane-spanning domain that anchors the protein to the plasma membrane and/or to intracellular organelles <cite>Molhoj2002 Brummell1997a</cite>. Membrane anchored EGases were first described in studies of the ''KORRIGAN'' (''KOR'') genes in<sup> </sup>''Arabidopsis thaliana'', which showed that they encode EGases that are required for normal cellulose synthesis or assembly.  Plants with mutant alleles of the ''KOR1'' gene are dwarfed, with decreased cellulose content and crystallinity <cite>Molhoj2002 Szyjanowicz2004 Takahashi2009</cite>.  The role of the Class A EGases in plants is not known.  However, the KOR proteins have been proposed to cleave sitosterol-b-glucoside primers from the growing cellulose polymer, or may have a role in editing incorrectly formed growing microfibrils <cite>Peng2002</cite>.   The catalytic domain of PttCel9A, a Class A [[GH9]] enzyme that is upregulated during secondary cell wall synthesis in ''Populus tremula x tremuloides'', has been biochemically characterized and shown to hydrolyse a narrow range of substrates ''in vitro'' including CMC, phosphoric acid swollen cellulose and cellulose oligosaccharides (DP≥5) <cite>Master2004 Rudsander2008</cite>.
-Previously, it was shown that during cell expansion, KOR1 is cycled from the plasma membrane through intracellular compartments, comprising both<sup> </sup>the Golgi apparatus and early endosomes; however the role of KOR1 in cellulose biosynthesis was unclear <cite>Robert2005</cite>.Recently, membrane-based split-ubiquitin assays and bimolecular fluorescence complementation have demonstrated a direct interaction between GH9A1/KOR1 and cellulose synthase isoforms (CESA1, CESA3 and CESA6) that comprise the primary cellulose synthase complex in Arabidopsis <cite>Vain2014</cite>.  GH9A1/KOR1 has also been shown to co-localize with the cellulose synthase complex at the plant plasma membrane <cite>Lei2014</cite>.   Two different mutations in KOR1 (kor1-1 and jiaoyao1 ) cause reduced motility of the cellulose synthase complex in the plasma membrane , suggesting a role for GH9A1/KOR1 in the cellulose synthesis and intracellular trafficking of the complexes <cite>Vain2014 Lei2014</cite>.  Interestingly, the jiaoyao1 mutation is caused by a point mutation (C to T), which results in an amino acid substitution (A577V) within the second GH9 active site signature motif, eliminating the endoglucanase activity of the enzyme.   This new data strongly suggests that the endoglucanase activity of GH9A1 is very  important, but not essential, for proper cellulose biosynthesis in plants <cite>Lei2014</cite>.
+Previously, it was shown that during cell expansion, KOR1 is cycled from the plasma membrane through intracellular compartments, comprising both<sup> </sup>the Golgi apparatus and early endosomes; however the role of KOR1 in cellulose biosynthesis was unclear <cite>Robert2005</cite>. Recently, membrane-based split-ubiquitin assays and bimolecular fluorescence complementation have demonstrated a direct interaction between GH9A1/KOR1 and cellulose synthase isoforms (CESA1, CESA3 and CESA6) that comprise the primary cellulose synthase complex in Arabidopsis <cite>Vain2014</cite>.  GH9A1/KOR1 has also been shown to co-localize with the cellulose synthase complex at the plant plasma membrane <cite>Lei2014</cite>.   Two different mutations in KOR1 (''kor1-1'' and ''jiaoyao1'' ) cause reduced motility of the cellulose synthase complex in the plasma membrane, suggesting a role for GH9A1/KOR1 in cellulose synthesis and intracellular trafficking of the complexes <cite>Vain2014 Lei2014</cite>.  Interestingly, the ''jiaoyao1'' mutation is caused by a point mutation (C to T), which results in an amino acid substitution (A577V) within the second GH9 active site signature motif, eliminating the endoglucanase activity of the enzyme.   This new data strongly suggests that the endoglucanase activity of GH9A1 is very important, but not essential, for proper cellulose biosynthesis in plants <cite>Lei2014</cite>.
 === Class B ===

@@ Line 59: / Line 59: @@
 #Szyjanowicz2004 pmid=14871312
 #Du2015 pmid=25716095
 </biblio>

@@ Line 15: / Line 15: @@
 === Class A ===
-The Class A EGases are integral type II membrane proteins with a [[GH9]] catalytic core that lack a canonical secretion signal sequence. These enzymes are predicted to have a high degree of N-glycosylation and a long amino-terminal extension with a membrane-spanning domain that anchors the protein to the plasma membrane and/or to intracellular organelles <cite>Molhoj2002 Brummell1997a</cite>. Membrane anchored EGases were first described in studies of the ''KORRIGAN'' (''KOR'') genes in<sup> </sup>''Arabidopsis thaliana'', which showed that they encode EGases that are required for normal cellulose synthesis or assembly.  Plants with mutant alleles of the ''KOR1'' gene are dwarfed, with decreased cellulose content and crystallinity <cite>Molhoj2002 Szyjanowicz2004 Takahashi2009</cite>.  The role of the Class A EGases in plants is not known.  However, the KOR proteins have been proposed to cleave sitosterol-b-glucoside primers from the growing cellulose polymer, or may have a role in editing incorrectly formed growing microfibrils <cite>Peng2002</cite>. More recently, it has been shown that during cell expansion, KOR1 is cycled from the plasma membrane through intracellular compartments, comprising both<sup> </sup>the Golgi apparatus and early endosomes; however the role of KOR1 in cellulose biosynthesis remains to be determined <cite>Robert2005</cite>.  The catalytic domain of PttCel9A, a Class A [[GH9]] enzyme that is upregulated during secondary cell wall synthesis in ''Populus tremula x tremuloides'', has been biochemically characterized and shown to hydrolyse a narrow range of substrates ''in vitro'' including CMC, phosphoric acid swollen cellulose and cellulose oligosaccharides (DP≥5) <cite>Master2004 Rudsander2008</cite>.
+The Class A EGases are integral type II membrane proteins with a [[GH9]] catalytic core that lack a canonical secretion signal sequence. These enzymes are predicted to have a high degree of N-glycosylation and a long amino-terminal extension with a membrane-spanning domain that anchors the protein to the plasma membrane and/or to intracellular organelles <cite>Molhoj2002 Brummell1997a</cite>. Membrane anchored EGases were first described in studies of the ''KORRIGAN'' (''KOR'') genes in<sup> </sup>''Arabidopsis thaliana'', which showed that they encode EGases that are required for normal cellulose synthesis or assembly.  Plants with mutant alleles of the ''KOR1'' gene are dwarfed, with decreased cellulose content and crystallinity <cite>Molhoj2002 Szyjanowicz2004 Takahashi2009</cite>.  The role of the Class A EGases in plants is not known.  However, the KOR proteins have been proposed to cleave sitosterol-b-glucoside primers from the growing cellulose polymer, or may have a role in editing incorrectly formed growing microfibrils <cite>Peng2002</cite>.   The catalytic domain of PttCel9A, a Class A [[GH9]] enzyme that is upregulated during secondary cell wall synthesis in ''Populus tremula x tremuloides'', has been biochemically characterized and shown to hydrolyse a narrow range of substrates ''in vitro'' including CMC, phosphoric acid swollen cellulose and cellulose oligosaccharides (DP≥5) <cite>Master2004 Rudsander2008</cite>.
 === Class B ===

@@ Line 12: / Line 12: @@
 == Plant [[GH9]] subfamilies ==
-In the model plant ''Arabidopsis thaliana'', 25 different [[GH9]] coding regions have been identified. Phylogenic analysis of the deduced amino acid sequences group the proteins into nine classes or three subfamilies <cite>Molhoj2002 Libertini2004 Urbanowicz2007 UrbanowiczBennett2007</cite>. Three distinct types of [[GH9]] proteins are present in plants. Class A proteins are membrane-anchored, Class B proteins are secreted, and Class C proteins are also secreted but contain a family 49 carbohydrate binding module (CBM49) <cite>Urbanowicz2007</cite>. Class A plant EGases have been reported to lack tryptophans corresponding to substrate binding at subsites -4, -3, and -2 in ''T. fusca ''Cel9A <cite>Master2004</cite>. Class C EGases are the only plant EGases to date that contain a tryptophan residue corresponding to the one in subsite -2 in TfCel9A <cite>Urbanowicz2007 Master2004</cite>.  This tryptophan has been shown to be important for hydrolysis in TfCel9A, and the enzyme retains less than 10% of its normal activity on polymeric cellulose substrates, and less than 1% of wild type activity on cellohexaose when the Trp is replaced by another amino acid <cite>Li2007 Master2004</cite>.
+In the model plant ''Arabidopsis thaliana'', 25 different [[GH9]] coding regions have been identified. Phylogenic analysis of the deduced amino acid sequences group the proteins into nine classes or three subfamilies <cite>Molhoj2002 Libertini2004 Urbanowicz2007 UrbanowiczBennett2007</cite>. Three distinct types of [[GH9]] proteins are present in plants. Class A proteins are membrane-anchored, Class B proteins are secreted, and Class C proteins are also secreted but contain a family 49 carbohydrate binding module (CBM49) <cite>Urbanowicz2007</cite>.   Recent bioinformatic studies suggest that the first gene duplication event that gave rise to the three plant GH9 sub-families took place prior to the divergence of angiosperms and gymnosperms about 300 million years ago, and most secondary duplication events occurred before the monocot/dicot divergence about 200 million years ago <cite>Du2015</cite>.    Class A plant EGases have been reported to lack tryptophans corresponding to substrate binding at subsites -4, -3, and -2 in ''T. fusca ''Cel9A <cite>Master2004</cite>. Class C EGases are the only plant EGases to date that contain a tryptophan residue corresponding to the one in subsite -2 in TfCel9A <cite>Urbanowicz2007 Master2004</cite>.  This tryptophan has been shown to be important for hydrolysis in TfCel9A, and the enzyme retains less than 10% of its normal activity on polymeric cellulose substrates, and less than 1% of wild type activity on cellohexaose when the Trp is replaced by another amino acid <cite>Li2007 Master2004</cite>.
 === Class A ===
@@ Line 56: / Line 56: @@
 #Brummell1997a pmid=9114071
 #Szyjanowicz2004 pmid=14871312
 </biblio>

@@ Line 15: / Line 15: @@
 === Class A ===
-The Class A EGases are integral type II membrane proteins with a [[GH9]] catalytic core that lack a canonical secretion signal sequence. These enzymes are predicted to have a high degree of N-glycosylation and a long amino-terminal extension with a membrane-spanning domain that anchors the protein to the plasma membrane and/or to intracellular organelles <cite>Molhoj2002 Brummell1997</cite>. Membrane anchored EGases were first described in studies of the ''KORRIGAN'' (''KOR'') genes in<sup> </sup>''Arabidopsis thaliana'', which showed that they encode EGases that are required for normal cellulose synthesis or assembly.  Plants with mutant alleles of the ''KOR1'' gene are dwarfed, with decreased cellulose content and crystallinity <cite>Molhoj2002 Szyjanowicz2004 Takahashi2009</cite>.  The role of the Class A EGases in plants is not known.  However, the KOR proteins have been proposed to cleave sitosterol-b-glucoside primers from the growing cellulose polymer, or may have a role in editing incorrectly formed growing microfibrils <cite>Peng2002</cite>. More recently, it has been shown that during cell expansion, KOR1 is cycled from the plasma membrane through intracellular compartments, comprising both<sup> </sup>the Golgi apparatus and early endosomes; however the role of KOR1 in cellulose biosynthesis remains to be determined <cite>Robert2005</cite>.  The catalytic domain of PttCel9A, a Class A [[GH9]] enzyme that is upregulated during secondary cell wall synthesis in ''Populus tremula x tremuloides'', has been biochemically characterized and shown to hydrolyse a narrow range of substrates ''in vitro'' including CMC, phosphoric acid swollen cellulose and cellulose oligosaccharides (DP≥5) <cite>Master2004 Rudsander2008</cite>.
+The Class A EGases are integral type II membrane proteins with a [[GH9]] catalytic core that lack a canonical secretion signal sequence. These enzymes are predicted to have a high degree of N-glycosylation and a long amino-terminal extension with a membrane-spanning domain that anchors the protein to the plasma membrane and/or to intracellular organelles <cite>Molhoj2002 Brummell1997a</cite>. Membrane anchored EGases were first described in studies of the ''KORRIGAN'' (''KOR'') genes in<sup> </sup>''Arabidopsis thaliana'', which showed that they encode EGases that are required for normal cellulose synthesis or assembly.  Plants with mutant alleles of the ''KOR1'' gene are dwarfed, with decreased cellulose content and crystallinity <cite>Molhoj2002 Szyjanowicz2004 Takahashi2009</cite>.  The role of the Class A EGases in plants is not known.  However, the KOR proteins have been proposed to cleave sitosterol-b-glucoside primers from the growing cellulose polymer, or may have a role in editing incorrectly formed growing microfibrils <cite>Peng2002</cite>. More recently, it has been shown that during cell expansion, KOR1 is cycled from the plasma membrane through intracellular compartments, comprising both<sup> </sup>the Golgi apparatus and early endosomes; however the role of KOR1 in cellulose biosynthesis remains to be determined <cite>Robert2005</cite>.  The catalytic domain of PttCel9A, a Class A [[GH9]] enzyme that is upregulated during secondary cell wall synthesis in ''Populus tremula x tremuloides'', has been biochemically characterized and shown to hydrolyse a narrow range of substrates ''in vitro'' including CMC, phosphoric acid swollen cellulose and cellulose oligosaccharides (DP≥5) <cite>Master2004 Rudsander2008</cite>.
 === Class B ===
-Class B proteins are the most common form of plant EGases and are associated with virtually all stages of plant growth and development.  These enzymes have a [[GH9]] catalytic domain and a signal sequence for ER targeting and secretion.  Different isoforms are expressed during fruit ripening, in abscission zones, in reproductive organ development, and in expanding cells <cite>Brummel1999 Brummel1997 Kalaitzis1999 Shani1997</cite>.  Numerous studies, especially in tomato, have also shown that many class B EGases are under hormonal control <cite>Catala1997 Brummell1997 Bonghi1998</cite>.
+Class B proteins are the most common form of plant EGases and are associated with virtually all stages of plant growth and development.  These enzymes have a [[GH9]] catalytic domain and a signal sequence for ER targeting and secretion.  Different isoforms are expressed during fruit ripening, in abscission zones, in reproductive organ development, and in expanding cells <cite>Brummel1999 Brummell1997 Kalaitzis1999 Shani1997</cite>.  Numerous studies, especially in tomato, have also shown that many class B EGases are under hormonal control <cite>Catala1997 Brummell1998 Bonghi1998</cite>.
 === Class C ===
@@ Line 36: / Line 36: @@
 #UrbanowiczBennett2007 pmid=17687051
 #Peng2002 pmid=11778054
 #Robert2005 pmid=16284310
 #Takahashi2009 pmid=19398462
@@ Line 54: / Line 53: @@
 #Urbanowicz2007 pmid=17322304
 #Li2007 pmid=17369336
-#Szyjanowicz2004  pmid=14871312
+#Brummell1998 pmid=9847104
 </biblio>

@@ Line 21: / Line 21: @@
 === Class C ===
-Plant Class C [[GH9]] enzymes are the least studied. These proteins are predicted to have a signal sequence followed by a [[GH9]] catalytic domain and a long carboxyl-terminal extension, which contains a CBM49 that has been shown to bind to crystalline cellulose ''in vitro'' <cite>Urbanowicz2007 UrbanowiczBennett2007</cite>.  CBMs are necessary for activity on crystalline substrates and may promote hydrolysis by increasing the local enzyme concentration at the substrate surface as well as modifying cellulose microfibril structure (for review see <cite>Boraston2004</cite>). The catalytic domain (CD) SlGH9C1 from tomato is promiscuous and can effectively hydrolyze artificial cellulosic polymers, cellulose oligosaccharides, and several plant cell wall polysaccharides <cite>Urbanowicz2007</cite>.  Nevertheless, the activity of the full length, modular enzyme has still not been characterized.  A Class C EGase from rice, OsCel9A, has been shown to be post-translationaly modified at the linker region to yield a 51 kDa [[GH9]] CD and a CBM49, and it was suggested that the cleavage is necessary for function <cite>YoshidaImaizumi2006</cite>.  The OsCel9A CD also displays a broad substrate range and was able to hydrolyze CMC, phosphoric acid-swollen cellulose, mixed linkage 1,3-1,4-ß-glucan, xylan, glucomannan, cellooligosaccharides (DP≥3) and 1,4-ß-xylohexaose <cite>YoshidaKomae2006</cite>. For Information regarding nomenclature of plant [[GH9]] enzymes please see Urbanowicz et al 2007 <cite>UrbanowiczBennett2007</cite>.
+Plant Class C [[GH9]] enzymes are the least studied. These proteins are predicted to have a signal sequence followed by a [[GH9]] catalytic domain and a long carboxyl-terminal extension, which contains a CBM49 that has been shown to bind to crystalline cellulose ''in vitro'' <cite>Urbanowicz2007 UrbanowiczBennett2007</cite>.  CBMs are necessary for activity on crystalline substrates and may promote hydrolysis by increasing the local enzyme concentration at the substrate surface as well as modifying cellulose microfibril structure (for review see <cite>Boraston2004</cite>). The catalytic domain (CD) SlGH9C1 from tomato is promiscuous and can effectively hydrolyze artificial cellulosic polymers, cellulose oligosaccharides, and several plant cell wall polysaccharides <cite>Urbanowicz2007</cite>.  Nevertheless, the activity of the full length, modular enzyme has still not been characterized.  A Class C EGase from rice, OsCel9A, has been shown to be post-translationaly modified at the linker region to yield a 51 kDa [[GH9]] CD and a CBM49, and it was suggested that the cleavage is necessary for function <cite>YoshidaImaizumi2006</cite>.  The OsCel9A CD also displays a broad substrate range and was able to hydrolyze CMC, phosphoric acid-swollen cellulose, mixed linkage 1,3-1,4-ß-glucan, xylan, glucomannan, cellooligosaccharides (DP≥4) and 1,4-ß-xylohexaose <cite>YoshidaKomae2006</cite>. For Information regarding nomenclature of plant [[GH9]] enzymes please see Urbanowicz et al 2007 <cite>UrbanowiczBennett2007</cite>.
 == References ==

@@ Line 6: / Line 6: @@
 ''Note:  This page is an extension of the [[Glycoside Hydrolase Family 9]] page, which is focussed on a key subgroup enzymes from plants.  Please see the main [[GH9]] page for full information on the functional and structural properties of these enzymes.''
-== Plant [[GH9]] Glycoside Hydrolases ==
+== Introduction ==
 Early reports described the existence of plant "cellulases" or EGases <cite>Hall1963</cite>.  Subsequently, cellulases have been shown to be associated with plant cell wall restructuring during cell expansion, the wall disassembly that accompanies processes such as fruit ripening and abscission (reviewed in <cite>Campillo1999 Rose1999 Molhoj2002</cite>) and cellulose biosynthesis <cite>Nicol1998 Lane2001 Sato2001</cite>. The amino acid sequences of the first plant "cellulases"/endo-ß-1,4-glucanases revealed that these enzymes belong to the CAZy family [[GH9]] glycoside hydrolases <cite>Henrissat1991</cite>.
 Most plant [[GH9]] [[glycoside hydrolases]] are endoglucanases ("cellulases", EC [{{EClink}}3.2.1.4 3.2.1.4]) with low or no activity on crystalline cellulose, but with discernible activity on soluble cellulose derivatives, including carboxymethyl cellulose (CMC), phosphoric acid swollen non-crystalline cellulose, and numerous plant polysaccharides including xylan, 1,3-1,4-ß-glucan, xyloglucan, and glucomannan <cite>Master2004 YoshidaKomae2006 Ohmiya2000 Woolley2001 Urbanowicz2007</cite>.  The inability of plant “cellulases” to hydrolyze crystaline cellulose is distinct from microbial cellulases, whose modular structure and synergistic action with  other enzymes facilitates effective degradation of crystalline cellulose.  ''In muro'', the substrates of plant cellulases may include xyloglucan, xylans, and non-crystalline cellulose, especially amorphous regions of cellulose where the microfibrils may be interwoven with xyloglucan.
-=== Plant [[GH9]] subfamilies ===
+== Plant [[GH9]] subfamilies ==
 In the model plant ''Arabidopsis thaliana'', 25 different [[GH9]] coding regions have been identified. Phylogenic analysis of the deduced amino acid sequences group the proteins into nine classes or three subfamilies <cite>Molhoj2002 Libertini2004 Urbanowicz2007 UrbanowiczBennett2007</cite>. Three distinct types of [[GH9]] proteins are present in plants. Class A proteins are membrane-anchored, Class B proteins are secreted, and Class C proteins are also secreted but contain a family 49 carbohydrate binding module (CBM49) <cite>Urbanowicz2007</cite>. Class A plant EGases have been reported to lack tryptophans corresponding to substrate binding at subsites -4, -3, and -2 in ''T. fusca ''Cel9A <cite>Master2004</cite>. Class C EGases are the only plant EGases to date that contain a tryptophan residue corresponding to the one in subsite -2 in TfCel9A <cite>Urbanowicz2007 Master2004</cite>.  This tryptophan has been shown to be important for hydrolysis in TfCel9A, and the enzyme retains less than 10% of its normal activity on polymeric cellulose substrates, and less than 1% of wild type activity on cellohexaose when the Trp is replaced by another amino acid <cite>Li2007 Master2004</cite>.
-==== Class A ====
+=== Class A ===
 The Class A EGases are integral type II membrane proteins with a [[GH9]] catalytic core that lack a canonical secretion signal sequence. These enzymes are predicted to have a high degree of N-glycosylation and a long amino-terminal extension with a membrane-spanning domain that anchors the protein to the plasma membrane and/or to intracellular organelles <cite>Molhoj2002 Brummell1997</cite>. Membrane anchored EGases were first described in studies of the ''KORRIGAN'' (''KOR'') genes in<sup> </sup>''Arabidopsis thaliana'', which showed that they encode EGases that are required for normal cellulose synthesis or assembly.  Plants with mutant alleles of the ''KOR1'' gene are dwarfed, with decreased cellulose content and crystallinity <cite>Molhoj2002 Szyjanowicz2004 Takahashi2009</cite>.  The role of the Class A EGases in plants is not known.  However, the KOR proteins have been proposed to cleave sitosterol-b-glucoside primers from the growing cellulose polymer, or may have a role in editing incorrectly formed growing microfibrils <cite>Peng2002</cite>. More recently, it has been shown that during cell expansion, KOR1 is cycled from the plasma membrane through intracellular compartments, comprising both<sup> </sup>the Golgi apparatus and early endosomes; however the role of KOR1 in cellulose biosynthesis remains to be determined <cite>Robert2005</cite>.  The catalytic domain of PttCel9A, a Class A [[GH9]] enzyme that is upregulated during secondary cell wall synthesis in ''Populus tremula x tremuloides'', has been biochemically characterized and shown to hydrolyse a narrow range of substrates ''in vitro'' including CMC, phosphoric acid swollen cellulose and cellulose oligosaccharides (DP≥5) <cite>Master2004 Rudsander2008</cite>.
-==== Class B ====
+=== Class B ===
 Class B proteins are the most common form of plant EGases and are associated with virtually all stages of plant growth and development.  These enzymes have a [[GH9]] catalytic domain and a signal sequence for ER targeting and secretion.  Different isoforms are expressed during fruit ripening, in abscission zones, in reproductive organ development, and in expanding cells <cite>Brummel1999 Brummel1997 Kalaitzis1999 Shani1997</cite>.  Numerous studies, especially in tomato, have also shown that many class B EGases are under hormonal control <cite>Catala1997 Brummell1997 Bonghi1998</cite>.
-==== Class C ====
+=== Class C ===
 Plant Class C [[GH9]] enzymes are the least studied. These proteins are predicted to have a signal sequence followed by a [[GH9]] catalytic domain and a long carboxyl-terminal extension, which contains a CBM49 that has been shown to bind to crystalline cellulose ''in vitro'' <cite>Urbanowicz2007 UrbanowiczBennett2007</cite>.  CBMs are necessary for activity on crystalline substrates and may promote hydrolysis by increasing the local enzyme concentration at the substrate surface as well as modifying cellulose microfibril structure (for review see <cite>Boraston2004</cite>). The catalytic domain (CD) SlGH9C1 from tomato is promiscuous and can effectively hydrolyze artificial cellulosic polymers, cellulose oligosaccharides, and several plant cell wall polysaccharides <cite>Urbanowicz2007</cite>.  Nevertheless, the activity of the full length, modular enzyme has still not been characterized.  A Class C EGase from rice, OsCel9A, has been shown to be post-translationaly modified at the linker region to yield a 51 kDa [[GH9]] CD and a CBM49, and it was suggested that the cleavage is necessary for function <cite>YoshidaImaizumi2006</cite>.  The OsCel9A CD also displays a broad substrate range and was able to hydrolyze CMC, phosphoric acid-swollen cellulose, mixed linkage 1,3-1,4-ß-glucan, xylan, glucomannan, cellooligosaccharides (DP≥3) and 1,4-ß-xylohexaose <cite>YoshidaKomae2006</cite>. For Information regarding nomenclature of plant [[GH9]] enzymes please see Urbanowicz et al 2007 <cite>UrbanowiczBennett2007</cite>.