Limited proteolysis of NADP-malate dehydrogenase from pea chloroplast by aminopeptidase K yields monomers. Evidence of proteolytic degradation of NADP-malate dehydrogenase during purification from pea

Avoiding Proteolysis during the Extraction and Purification of Active Plant Enzymes

The aim of this article is to discuss approaches to diagnose and prevent unwanted proteolysis during extraction and isolation of active enzymes from plant tissues. Enzymes are protein catalysts that require great care during sample processing in order to ensure that they remain intact and fully active. Preventing artifactual enzyme modifications ex planta is of utmost importance in order to obtain biologically relevant data. This is particularly problematic following enzyme extraction from plant tissues, which relative to microbes or animals contain relatively low protein amounts coupled with high concentrations of vacuolar proteases. Although cytoplasmic enzymes are not directly accessible to vacuolar proteases owing their physical segregation into different subcellular compartments, this compartmentation is destroyed during cell lysis. Unwanted proteolysis by endogenous proteases is an insidious problem because in many cases the enzyme of interest is only partially degraded and retains catalytic activity. This can not only lead to erroneous conclusions about an enzyme's size, subunit structure and post-translational modifications, but can also result in striking changes to its kinetic and regulatory (i.e. allosteric) properties. Furthermore, the routine addition of class-specific protease inhibitors and/or commercially available (and expensive) protease inhibitor cocktails to extraction and purification buffers does not necessarily preclude partial proteolysis of plant enzymes by endogenous proteases. When antibodies are available, plant scientists are advised to employ immunoblotting to diagnose potential in vitro proteolytic truncation of the enzymes that they wish to characterize, as well as to test the effectiveness of specific protease inhibitors in overcoming this recurrent issue.

Arabidopsis UDP-sugar pyrophosphorylase: evidence for two isoforms

Arabidopsis UDP-sugar pyrophosphorylase (AtUSP, EC 2.7.7.64) is a broad substrate pyrophosphorylase that exhibits activity with GlcA-1-P, Gal-1-P and Glc-1-P. Immunoblots using polyclonal antibodies raised to recombinant AtUSP demonstrated the presence of two USP isoforms of approximately 70 kDa (USP1) and 66 kDa (USP2) in crude extracts of Arabidopsis tissues. The 66 kDa isoform was not the result of proteolytic cleavage of USP1 during extraction. Trypsin digestion of bands on SDS gels corresponding to the location of the two isoforms followed by tandem mass spectrometry confirmed that USP peptides were present in both bands. Both USP isoforms were detected in the cytosol as determined by immunoblots of cellular fractions obtained by differential centrifugation. However, some USP1 was also detected in the microsomal fraction. Immunoprecipitation assays demonstrated that AtUSP antibodies removed USP activity (UDP-GlcA-->GlcA-1-P) measured in floret extracts. These results indicate that USP is the only pyrophosphorylase that utilizes UDP-GlcA as a substrate and suggest that it serves as the terminal enzyme of the myo-inositol oxidation pathway.

The dimer contact area of sorghum NADP-malate dehydrogenase: role of aspartate 101 in dimer stability and catalytic activity

During thioredoxin-mediated activation of chloroplastic NADP-malate dehydrogenase, a homodimeric enzyme, the interaction between subunits is known to be loosened but maintained. A modeling of the 3D structure of the protein identified Asp-101 as being potentially involved in the association between subunits through an electrostatic interaction. Indeed, upon site-directed substitution of Asp-101 by an asparagine, the mutated enzyme behaved mainly as a monomer. The mutation strongly affected the catalytical efficiency of the enzyme. The now available 3D structure of the enzyme shows that Asp-101 is protruding at the dimer interface, interacting with Arg-268 of the neighbouring subunit.

An internal cysteine is involved in the thioredoxin-dependent activation of sorghum leaf NADP-malate dehydrogenase

The chloroplastic NADP-malate dehydrogenase is activated by thiol/disulfide interchange with reduced thioredoxins. Previous experiments showed that four cysteines located in specific N- and carboxyl-terminal extensions were implicated in this process, leading to a model where no internal cysteine was involved in activation. In the present study, the role of the conserved four internal cysteines was investigated. Surprisingly, the mutation of cysteine 207 into alanine yielded a protein with accelerated activation time course, whereas the mutations of the three other internal cysteines into alanines yielded proteins with unchanged activation kinetics. These results suggested that cysteine 207 might be linked in a disulfide bridge with one of the four external cysteines, most probably with one of the two amino-terminal cysteines whose mutation similarly accelerates the activation rate. To investigate this possibility, mutant malate dehydrogenases (MDHs) where a single amino-terminal cysteine was mutated in combination with the mutation of both carboxyl-terminal cysteines were produced and purified. The C29S/C365A/C377A mutant MDH still needed activation by reduced thioredoxin, while the C24S/C365A/C377A mutant MDH exhibited a thioredoxin-insensitive spontaneous activity, leading to the hypothesis that a Cys24-Cys207 disulfide bridge might be formed during the activation process. Indeed, an NADP-MDH where the cysteines 29, 207, 365, and 377 are mutated yielded a permanently active enzyme very similar to the previously created permanently active C24S/C29S/C365A/C377A mutant. A two-step activation model involving a thioredoxin-mediated disulfide isomerization at the amino terminus is proposed.

Phosphorylation of phosphoenolpyruvate carboxykinase in plants. Studies in plants with C4 photosynthesis and Crassulacean acid metabolism and in germinating seeds

We have previously shown that phosphoenolpyruvate carboxykinase (PEPCK) is phosphorylated in vivo in the cotyledons of darkened cucumber seedlings and that phosphorylation is reversed by light [Walker and Leegood (1995) FEBS Lett. 362, 70-74]. In this study the molecular mass of PEPCK was estimated in a range of gluconeogenic seedlings and in leaves of C4 plants and plants with Crassulacean acid metabolism (CAM). Phosphorylation of PEPCK was studied in these plants by feeding tissues with [32P]Pi and assessing phosphorylation by SDS/PAGE and autoradiography of either total proteins or of immunoprecipitated protein. In gluconeogenic seedlings and most CAM plants PEPCK had a molecular mass of 74 kDa, whereas in C4 grasses the molecular mass of PEPCK was always smaller and varied from 67-71 kDa. In all gluconeogenic seedlings and leaves of CAM plants PEPCK was phosphorylated, but it was not phosphorylated in all species of C4 grasses studied. In CAM plants, phosphorylation of PEPCK occurred at night and dephosphorylation occurred during the day. In C4 grasses phosphorylation occurred when leaves were darkened and the enzyme was dephosphorylated following illumination, but it was only phosphorylated in those plants with larger (71 kDa) molecular mass forms of PEPCK.

Membrane topologies of the TolQ and TolR proteins of Escherichia coli: inactivation of TolQ by a missense mutation in the proposed first transmembrane segment

The TolQ and TolR proteins of Escherichia coli are required for the uptake of group A colicins and for infection by filamentous phages. Their topology in the cytoplasmic membrane was determined by cleavage with aminopeptidase K, proteinase K, and trypsin in spheroplasts and cell lysates. From the results obtained, it is proposed that the N terminus of TolQ is located in the periplasm and that it contains three transmembrane segments (residues 9 to 36, 127 to 159, and 162 to 191), a small periplasmic loop, and two large portions in the cytoplasm. The N terminus of TolR is located in the cytoplasm and is followed by a transmembrane segment (residues 21 to 40), and the remainder of the protein is located in the periplasm. A tolQ mutant, which rendered cells resistant to group A colicins and sensitive to cholate, had alanine 13 replaced by glycine and was lacking serine 14 in the first transmembrane segment. The membrane topologies of TolQ and TolR are similar to those proposed for ExbB and ExbD, respectively, which is consistent with the partial functional substitution between ExbB and TolQ and between ExbD and TolR. The amino acid sequences of these proteins display the highest homology in the transmembrane segments, which indicates that the membrane-spanning regions play an important role in the activities of the proteins.

A thioredoxin-independent fully active NADP-malate dehydrogenase obtained by site-directed mutagenesis

A triple cysteine mutant of sorghum leaf NADP-malate dehydrogenase has been constructed by site-directed mutagenesis, combining the previously obtained mutation of the two N-terminal cysteines with the mutation of the most internal of the two C-terminal cysteines. The construct, over-expressed in E. coli, yielded an always active, dithiol-insensitive enzyme. It can be concluded that the dithiol activation of the unmodified enzyme involves a maximum of two different disulfides per subunit, and that none of the mutated cysteines is implicated in catalysis.