Analysis of allosteric effector binding sites of potato ADP-glucose pyrophosphorylase through reverse genetics

ADP-glucose pyrophosphorylase (AGPase) is a key regulatory enzyme of bacterial glycogen and plant starch synthesis as it controls carbon flux via its allosteric regulatory behavior. Unlike the bacterial enzyme that is composed of a single subunit type, the plant AGPase is a heterotetrameric enzyme (alpha2beta2) with distinct roles for each subunit type. The large subunit (LS) is involved mainly in allosteric regulation through its interaction with the catalytic small subunit (SS). The LS modulates the catalytic activity of the SS by increasing the allosteric regulatory response of the hetero-oligomeric enzyme. To identify regions of the LS involved in binding of effector molecules, a reverse genetics approach was employed. A potato (Solanum tuberosum L.) AGPase LS down-regulatory mutant (E38A) was subjected to random mutagenesis using error-prone polymerase chain reaction and screened for the capacity to form an enzyme capable of restoring glycogen production in glgC(-) Escherichia coli. Dominant mutations were identified by their capacity to restore glycogen production when the LS containing only the second site mutations was co-expressed with the wild-type SS. Sequence analysis showed that most of the mutations were decidedly nonrandom and were clustered at conserved N- and C-terminal regions. Kinetic analysis of the dominant mutant enzymes indicated that the K(m) values for cofactor and substrates were comparable with the wild-type AGPase, whereas the affinities for activator and inhibitor were altered appreciably. These AGPase variants displayed increased resistance to P(i) inhibition and/or greater sensitivity toward 3-phosphoglyceric acid activation. Further studies of Lys-197, Pro-261, and Lys-420, residues conserved in AGPase sequences, by site-directed mutagenesis suggested that the effectors 3-phosphoglyceric acid and P(i) interact at two closely located binding sites.

In nonhepatic cells, cholesterol 7alpha-hydroxylase induces the expression of genes regulating cholesterol biosynthesis, efflux, and homeostasis

CHO cells expressing the liver-specific gene product cholesterol-7alpha-hydroxylase showed a 6-fold increase in the biosynthesis of [(14)C]cholesterol from [(14)C]acetate, as well as increased enzymatic activities of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and squalene synthase. Cells expressing cholesterol-7alpha-hydroxylase contained less sterol response element-binding protein 1 (SREBP1) precursor, whereas the cellular content of mature SREBP1, as well as the mRNAs of cholesterol biosynthetic genes (HMG-CoA reductase and squalene synthase), were all increased approximately 3-fold. Cells expressing cholesterol-7alpha-hydroxylase displayed greater activities of luciferase reporters containing the SREBP-dependent promoter elements derived from HMG-CoA reductase and farnesyl diphosphate synthase, in spite of accumulating significantly more free and esterified cholesterol and 7alpha-hydroxycholesterol. While cells expressing cholesterol-7alpha-hydroxylase displayed increased SREBP-dependent transcription, sterol-mediated repression of SREBP-dependent transcription by LDL-cholesterol and exogenous oxysterols was similar in both cell types. Cells expressing cholesterol-7alpha-hydroxylase displayed greater rates of secretion of cholesterol as well as increased expression of the ABC1 cassette protein mRNA. Adding 25-hydroxycholesterol to the culture medium of both cell types increased the expression of ABC1 cassette protein mRNA. The combined data suggest that in nonhepatic CHO cells multiple regulatory processes sensitive to cellular sterols act independently to coordinately maintain cellular cholesterol homeostasis.

Engineering starch for increased quantity and quality

The characterization and production of starch variants from mutation studies and transgene technology has been invaluable for our understanding of the synthesis of the starch granule. The knowledge gained has allowed for genetic manipulation of the starch biosynthetic pathway in plants. This in vivo approach can be used to generate novel starches and diminishes the need for post-harvest chemically and enzymatically treated starches. Thus, the modification of the starch biosynthetic pathway is a plausible means by which starches with novel properties and applications can be created.