In Vitro Synthesis and Processing of Wheat alpha-Amylase : TRANSLATION OF GIBBERELLIC ACID-INDUCED WHEAT ALEURONE LAYER RNA BY WHEAT GERM AND XENOPUS LAEVIS OOCYTE SYSTEMS

Xenopus oocytes as a heterologous expression system for plant proteins

The Xenopus oocyte is a robust and convenient system for the transient expression of many different animal proteins and it has recently been demonstrated that oocytes can also translate, process, and target plant proteins. This expression system can also be used to clone genes, characterize function, and study posttranslational processing of proteins. Here we describe the methodology for the expression of plant proteins, in particular membrane proteins, in Xenopus oocytes.

Functional expression of a plant plasma membrane transporter in Xenopus oocytes

A full-length cDNA clone for the H+/hexose co-transporter (STP1) from Arabidopsis thaliana has been transcribed in vitro and the mRNA injected into Xenopus oocytes. Under optimized conditions, oocytes injected with the STP1 mRNA accumulated 3-O-[methyl-14C]glucose at rates of more than a 1000-fold greater than water-injected control oocytes. A hexose-elicited depolarization of the oocyte membrane potential was demonstrated, and uptake was shown to be stimulated by low external pH, confirming the activity of a H+/hexose co-transport system. This is the first example of the functional expression of a plant membrane transporter in oocytes.

Biosynthesis, primary structure and molecular cloning of snowdrop (Galanthus nivalis L.) lectin

Poly(A)-rich RNA isolated from ripening ovaries of snowdrop (Galanthus nivalis L.) yielded a single 17-kDa lectin polypeptide upon translation in a wheat-germ cell-free system. This lectin was purified by affinity chromatography. Translation of the same RNA in Xenopus leavis oocytes revealed a lectin polypeptide which was about 2 kDa smaller than the in vitro synthesized precursor, suggesting that the oocyte system had removed a 2-kDa signal peptide. A second post-translational processing step was likely to be involved since both the in vivo precursor and the Xenopus translation products were about 2 kDa larger than the mature lectin polypeptide. This hypothesis was confirmed by the structural analysis of the amino acid sequence of the mature protein and the cloned mRNA. Edman degradation and carboxypeptidase Y digestion of the mature protein, and structural analysis of the peptides obtained after chemical cleavage and modification, allowed determination of the complete 105 amino acid sequence of the snowdrop lectin polypeptide. Comparison of this sequence with the deduced amino acid sequence of a lectin cDNA clone revealed that besides the mature lectin polypeptide, the lectin mRNA also encoded a 23 amino acid signal-sequence and a C-terminal extension of 29 amino acids, which confirms the results from in vitro translation experiments.

Biosynthesis and intracellular processing of carbonic anhydrase in Chlamydomonas reinhardtii

Carbonic anhydrase (CA) of Chlamydomonas reinhardtii is a glycoprotein of 35 kDa which is localized outside the plasma membrane. The activity of CA was increased when the CO2 concentration during photoautotrophic growth was decreased to air level. After decreasing the CO2 concentration from 4% to 0.04%, several polypeptides including CA were induced continuously or transiently. To investigate the biosynthesis and intracellular processing of CA, the cells of wall-less mutant CW-15, which secretes CA into the culture medium, were pulse-labeled with radioactive arginine, chased, and radioactive proteins were immunoprecipitated with anti-CA serum. A 42-kDa polypeptide with isoelectric point (pI) of 7.1-7.3 was first synthesized. Within 5 min the molecular mass of this polypeptide was decreased to 35 kDa and it was then secreted into the culture medium within 30 min. This indicates that the former is the precursor form and the latter the mature form of CA. The primary translation product from poly(A)-rich RNA in a cell-free reticulocyte lysate system from a rabbit was a 38-kDa polypeptide. This was cotranslationally converted into the 42-kDa precursor in vitro in the presence of dog pancreatic microsomal membranes. As the 42-kDa precursor had a high affinity to concanavalin A, it was assumed to have a high-mannose-type oligosaccharide. The mature enzyme had a pI of 6.1-6.2 and was composed of more than two isoforms, which had a complex-type oligosaccharide with low affinity to concanavalin A. Chemical deglycosylation of the mature enzyme by trifluoromethanesulfonic acid indicated that the molecular mass of the polypeptide moiety was 32 kDa and the difference between this and the primary translation product suggests that cleavage of the polypeptide occurs during its biosynthesis.

Biosynthesis of alpha-Amylase in Vigna mungo Cotyledon

In vitro translation of RNA extracted from Vigna mungo cotyledons showed that alpha-amylase is synthesized as a polypeptide with a molecular mass of 45,000, while cotyledons contain a form of alpha-amylase with a molecular mass of 43,000. To find out whether the 45,000 molecular mass polypeptide is a precursor to the 43,000 found in vivo, the cell free translation systems were supplemented with canine microsomal membrane; when mRNA was translated in the wheat germ system supplemented with canine microsomes, the 45,000 molecular mass form was not processed to a smaller form but the precursor form was partly processed in the membrane-supplemented reticulocyte lysate system. When V. mungo RNA was translated in Xenopus oocyte system, only the smaller form (molecular mass 43,000) was detected. Involvement of contranslational glycosylation in the maturating process of the alpha-amylase was ruled out because there was no effect of tunicamycin, and the polypeptide was resistant to endo-beta-H or endo-beta-D digestion. We interpret these results to mean that the 45,000 molecular mass form is a precursor with a signal peptide or transit sequence, and that the 43,000 molecular mass is the mature form of the protein.

Cell cycle regulation of tubulin RNA level, tubulin protein synthesis, and assembly of microtubules in Physarum

The temporal relationship between tubulin expression and the assembly of the mitotic spindle microtubules has been investigated during the naturally synchronous cell cycle of the Physarum plasmodium. The cell cycle behavior of the tubulin isoforms was examined by two-dimensional gel electrophoresis of proteins labeled in vivo and by translation of RNA in vitro. alpha 1-, alpha 2-, beta 1-, and beta 2-tubulin synthesis increases coordinately until metaphase, and then falls, with beta 2 falling more rapidly than beta 1. Nucleic acid hybridization demonstrated that alpha- and beta-tubulin RNAs accumulate coordinately during G2, peaking at metaphase. Quantitative analysis demonstrated that alpha-tubulin RNA increases with apparent exponential kinetics, peaking with an increase over the basal level of greater than 40-fold. After metaphase, tubulin RNA levels fall exponentially, with a short half-life (19 min). Electron microscopic analysis of the plasmodium showed that the accumulation of tubulin RNA begins long before the polymerization of mitotic spindle microtubules. By contrast, the decay of tubulin RNA after metaphase coincides with the depolymerization of the spindle microtubules.

Cell type-dependent expression of tubulins in Physarum

Three alpha-tubulins and two beta-tubulins have been resolved by two-dimensional gel electrophoresis of whole cell lysates of Physarum myxamoebae or plasmodia. Criteria used to identify the tubulins included migration on two-dimensional gels with myxamoebal tubulins purified by self-assembly into microtubules in vitro, peptide mapping with Staphylococcus V8 protease and with chymotrypsin, immunoprecipitation with a monoclonal antibody specific for beta-tubulin, and, finally, hybrid selection of specific mRNA by cloned tubulin DNA sequences, followed by translation in vitro. Differential expression of the Physarum tubulins was observed. The alpha 1- and beta 1-tubulins were detected in both myxamoebae and plasmodia; alpha 2 and beta 2 were detected only in plasmodia, alpha 3 was detected only in the myxamoebal phase, and may be specific to the flagellate. Observation of more tubulin species in plasmodia than in myxamoebae was remarkable; the only microtubules detected in plasmodia are those of the mitotoic spindle, whereas myxamoebae display cytoplasmic, centriolar, flagellar, and mitotic-spindle microtubules. In vitro translation of myxamoebal and plasmodial RNAs indicated that there are distinct mRNAs, and therefore probably separate genes, for the alpha 1-, alpha 2-, beta 1-, and beta 2-tubulins. Thus, the different patterns of tubulin expression in myxamoebae and plasmodia reflect differential expression of tubulin genes.

In vivo synthesis and processing of cereal lectins

The synthesis and processing of cereal lectins was followed in vivo. The initial translation products of lectin genes are higher molecular weight (28 K) precursors, which are post-translationally processed in a single step into authentic lectin polypeptides (23 K). The conversion of precursor into mature product is a rather slow process (the precursor has a half life of 36 min) and is apparently not a prerequisite for biological activity since the precursor exhibits sugar binding activity. Because of the striking resemblances between the processing of cereal lectins and vectorial processing of cytoplasmatically made chloroplast, mitochondrial and glyoxysomal proteins, vectorial processing of cereal lectins might be a means of transporting these proteins through a membrane into an extra-cytoplasmic compartment.

The role of the endoplasmic reticulum in the synthesis and transport of α-amylase in barley aleurone layers

The subcellular site of α-amylase (EC 1.6.2.1) synthesis and transport was studied in barley aleurone layers incubated in the presence or absence of gibberellic acid (GA3). Using [(35)S]methionine as a marker, the site of amino-acid incorporation into organelles isolated from aleurone layers incubated with and without GA3 was determined following purification by isopycnic sucrose-density-gradient centrifugation. Incorporation of radioactivity into trichloroacetic-acid-insoluble proteins was greatest in those fractions exhibiting activity of an endoplasmic reticulum (ER) marker enzyme. Further fractionation of densitygradient fractions by sodium-dodecyl-sulfate polyacrylamide-gel electrophoresis showed that a major portion of the radioactivity in the ER fractions was present in a protein co-migrating with marker α-amylase. This protein was identified as authentic α-amylase by immunoadsorbent chromatography and affinity chromatography. The newly synthesized α-amylase associated with the ER was shown to be sequenstered within the lumen of the ER by experiments which showed that the enzyme was resistant to proteolytic degradation. The labelled α-amylase sequestered in the ER can be chased from this organelle when tissue is incubated in unlabelled methionine following a 1-h pulse of labelled methionine. The isoenzymic forms of α-amylase found in tissue homogenates and incubation media of aleurone layers incubated with and without GA3 were characterized after chromatography on diethylaminoethyl cellulose. In homogenates of GA3-treated aleurone layers, five peaks of α-amylase activity were detected, while in homogenates of aleurone layers incubated with-out GA3 only three peaks of activity were found. In incubation media, four isoenzymes were found after GA3 treatment and two were found after incubation without GA3. We conclude that at least five α-amylase isoenzymes are synthesized by the ER of barley aleurone layers and that this membrane system is involved in the sequestration and transport of four of these isoenzymes.