A bacterial glucanotransferase can replace the complex maltose metabolism required for starch to sucrose conversion in leaves at night.
Controlled conversion of leaf starch to sucrose at night is essential for the normal growth of Arabidopsis. The conversion involves the cytosolic metabolism of maltose to hexose phosphates via an unusual, multidomain protein with 4-glucanotransferase activity, DPE2, believed to transfer glucosyl moieties to a complex heteroglycan prior to their conversion to hexose phosphate via a cytosolic phosphorylase. The significance of this complex pathway is unclear; conversion of maltose to hexose phosphate in bacteria proceeds via a more typical 4-glucanotransferase that does not require a heteroglycan acceptor. It has recently been suggested that DPE2 generates a heterogeneous series of terminal glucan chains on the heteroglycan that acts as a "glucosyl buffer" to ensure a constant rate of sucrose synthesis in the leaf at night. Alternatively, DPE2 and/or the heteroglycan may have specific properties important for their function in the plant. To distinguish between these ideas, we compared the properties of DPE2 with those of the Escherichia coli glucanotransferase MalQ. We found that MalQ cannot use the plant heteroglycan as an acceptor for glucosyl transfer. However, experimental and modeling approaches suggested that it can potentially generate a glucosyl buffer between maltose and hexose phosphate because, unlike DPE2, it can generate polydisperse malto-oligosaccharides from maltose. Consistent with this suggestion, MalQ is capable of restoring an essentially wild-type phenotype when expressed in mutant Arabidopsis plants lacking DPE2. In light of these findings, we discuss the possible evolutionary origins of the complex DPE2-heteroglycan pathway.
From the John Innes Centre, Norwich Research Park, Norwich NR4 7UH, United Kingdom.
Christian
C
Ruzanski
Julia
J
Smirnova
Martin
M
Rejzek
Darrell
D
Cockburn
Henriette L
HL
Pedersen
Marilyn
M
Pike
William G T
WG
Willats
Birte
B
Svensson
Martin
M
Steup
Oliver
O
Ebenhöh
Alison M
AM
Smith
Robert A
RA
Field
D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development; D002021:Buffers; D003600:Cytosol / Q000378:metabolism; D003624:Darkness; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism; D019143:Evolution, Molecular; D005964:Glucosyltransferases / Q000737:chemistry / Q000378:metabolism*; D008320:Maltose / Q000378:metabolism*; D055432:Metabolomics; D009154:Mutation / Q000235:genetics; D009844:Oligosaccharides / Q000378:metabolism; D010641:Phenotype; D018515:Plant Leaves / Q000378:metabolism*; D030821:Plants, Genetically Modified; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000302:isolation & purification; D013213:Starch / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D013395:Sucrose / Q000378:metabolism*
D002021:Buffers; D029968:Escherichia coli Proteins; D009844:Oligosaccharides; D011994:Recombinant Proteins; D013395:Sucrose; D008320:Maltose; D013213:Starch; D005964:Glucosyltransferases
Carbohydrate Metabolism; Computer Modeling; Glucanotransferase; Leaf Cell; Maltose Metabolism; Metabolic Regulation; Oligosaccharide; Plant Biochemistry; Starch Degradation