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Abstract
Quality Protein Maize (QPM) is a hard-endosperm version of the high-lysine opaque2 (o2) maize (Zea mays) mutant, but the genes involved in modification of the soft o2 endosperm are largely unknown. Pyrophosphate-dependent fructose-6-phosphate 1-phosphotransferase (PFP) catalyzes the ATP-independent conversion of fructose-6-phosphate to fructose-1,6-bisphosphate in glycolysis. We found a large increase in transcript and protein levels of the α-regulatory subunit of PFP (PFPα) in QPM endosperm. In vitro enzyme assays showed a significant increase in forward PFP activity in developing endosperm extracts of QPM relative to the wild type and o2. An expressed retrogene version of PFPα of unknown function that was not up-regulated in QPM was also identified. The elevated expression levels of a number of ATP-requiring heat shock proteins (Hsps) in o2 endosperm are ameliorated in QPM. PFPα is also coinduced with Hsps in maize roots in response to heat, cold, and the unfolded protein response stresses. We propose that reduced ATP availability resulting from the generalized Hsp response in addition to the reduction of pyruvate, orthophosphate dikinase activity in o2 endosperm is compensated in part by increased PFP activity in QPM.
Highlights
Quality Protein Maize (QPM) is a hard-endosperm version of the high-lysine opaque2 (o2) maize (Zea mays) mutant, but the genes involved in modification of the soft o2 endosperm are largely unknown
We propose that reduced ATP availability resulting from the generalized heat shock proteins (Hsps) response in addition to the reduction of pyruvate, orthophosphate dikinase activity in o2 endosperm is compensated in part by increased PFP activity in QPM
We describe work to characterize the physiological significance of the increase in pyrophosphate-dependent fructose-6phosphate 1-phosphotransferase (PFPa), which may lead to increased glycolytic flux, which may in turn play a role in amelioration of the o2 phenotype in QPM endosperm
Summary
Quality Protein Maize (QPM) is a hard-endosperm version of the high-lysine opaque (o2) maize (Zea mays) mutant, but the genes involved in modification of the soft o2 endosperm are largely unknown. We found a large increase in transcript and protein levels of the a-regulatory subunit of PFP (PFPa) in QPM endosperm. The elevated expression levels of a number of ATP-requiring heat shock proteins (Hsps) in o2 endosperm are ameliorated in QPM. We propose that reduced ATP availability resulting from the generalized Hsp response in addition to the reduction of pyruvate, orthophosphate dikinase activity in o2 endosperm is compensated in part by increased PFP activity in QPM. The maize opaque (o2) mutant has double the wild-type levels of Lys and Trp (Mertz et al, 1964), since it accumulates low levels of zein proteins and high levels of nonzein proteins that impart a good balance of all essential amino acids. Since the 27-kD g-zein is thought to initiate protein body formation, it has been suggested that this increase causes the observed elevation in protein body number in QPM (Lopes and Larkins, 1995; Moro et al, 1995), which in turn increases protein cross-linking and restores kernel hardness
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