Expression of phaseolin cDNA genes in yeast under control of natural plant DNA sequences

Pulsed-field gel electrophoresis analysis of the phaseolin locus region in Phaseolus vulgaris

Phaseolin is the major seed storage protein of common bean (Phaseolus vulgaris L.). It is encoded by a small multigene family of 6-9 genes that are clustered in a single complex locus (Phs). We have constructed a long-range restriction map of the phaseolin genomic region, including the Phs locus and two flanking marker loci, D1861 and Bng060. Using a combination of high molecular weight DNA isolation, one- and two-dimensional pulsed-field gel electrophoresis of single and double restriction digests followed by Southern hybridization, and PCR analysis of individual fragments, we found that: (i) the maximum size of the Phs locus is 190 kb, (ii) the Phs locus may have increased in size during the evolution of P. vulgaris, (iii) the genomic region marked by D1861-Phs-Bng060 spans 5 cM, which corresponds to a maximum of 1.9 Mb, and (iv) the Phs locus could be oriented with respect to the two adjacent markers. Further progress in determining the gene arrangement in the Phs locus will require cloning and analysis of large DNA fragments containing phaseolin genes via BAC libraries. Key words : multigene family, physical distance, genome mapping, seed protein.

The Bean Seed Storage Protein [beta]-Phaseolin Is Synthesized, Processed, and Accumulated in the Vacuolar Type-II Protein Bodies of Transgenic Rice Endosperm

The seed storage protein [beta]-phaseolin of the common bean (Phaseolus vulgaris L.) was expressed in the endosperm of transgenic rice (Oryza sativa L.) plants. The 5.1- or 1.8-kb promoter fragment of the rice seed storage protein glutelin Gt1 gene was fused transcriptionally to either the genomic or cDNA coding sequence of the [beta]-phaseolin gene. The highest quantity of phaseolin estimated by enzyme-linked immunosorbent assay was 4.0% of the total endosperm protein in the transgenic rice seeds. The phaseolin trait was segregated as a single dominant trait with a positive gene dosage effect and was stably inherited through three successive generations. Both phaseolin genomic and cDNA coding sequences were used to synthesize four isoforms of mature phaseolin protein with apparent molecular masses of 51, 48, 47, and 45 kD. Enzyme deglycosylation experiments indicated that the 51-kD form contains high-mannose N-glycans; the 48- and 47-kD forms have further modified N-glycans; and the 45-kD form is a nonglycosylated protein. Immunolabeling studies using light and electron microscopy demonstrated that phaseolin accumulates primarily in the vacuolar type-II protein bodies located at the periphery of the endosperm near the aleurone layer. We discuss the implications of these results on nutritional improvement of rice grains.

Regulatory sequences for expressing genes in oomycete fungi

Promoter and terminator sequences from a range of species were tested for activity in the oomycetes, a group of lower fungi that bear an uncertain taxonomic affinity to other organisms and in which little is known of the sequences required for transcription. Transient assays, using the reporter gene beta-glucuronidase (GUS), were used to examine the function of these promoters and terminators in the plant pathogens Phytophthora infestans and P. megasperma f. sp. glycinea, and in the saprophytic water mold, Achlya ambisexualis. Oomycete promoters, isolated from the ham34 and hsp70 genes of Bremia lactucae and the actin gene of P. megasperma f. sp. glycinea, resulted in high levels of GUS accumulation in each of the three oomycetes. In contrast, little or no activity was detected when promoters from higher fungi (four ascomycetes and one basidiomycete), plants, and animals were tested. The terminator from the ham34 gene resulted in much higher levels of GUS accumulation than did others, although an oomycete terminator was not absolutely required for expression. Transcript mapping of RNA from stable transformants confirmed accurate initiation from the B. lactucae hsp70 promoter and termination within 3' ham34 sequences in P. infestans. Our results indicate that the transcriptional machinery of the oomycetes differs significantly from that of the higher fungi, but that enough conservation exists within the class to allow vectors developed from one oomycete species to be used for others.

Differential accumulation of four phaseolin glycoforms in transgenic tobacco

An intron-less phaseolin gene was used to express phaseolin polypeptides in transgenic tobacco plants. The corresponding amounts of phaseolin immunoreactive polypeptides and mRNA were similar to those found in plants transformed with a bean genomic DNA sequence that encodes an identical beta-phaseolin subunit. These results justified the use of the intron-less gene for engineering of the phaseolin protein by oligonucleotide-directed mutagenesis. Each and both of the two Asn residues that serve as glycan acceptors in wild-type phaseolin were modified to prevent N-linked glycosylation. Wild-type (beta wti-) and mutant phaseolin glycoforms (beta dgly1, beta dgly2 and beta dgly1,2) were localized to the protein body matrix by immunogold microscopy. Although quantitative slot-blot hybridization analysis showed similar levels of phaseolin mRNA in transgenic seed derived from all constructs, seed from the beta dgly1 and beta dgly2 mutations contained only 41% and 73% of that expressed from the wild-type control; even less (23%) was present in seed of plants transformed with the phaseolin beta dgly1,2 gene. Additionally, the profile of 25-29 kDa processed peptides was different for each of the glycoforms, indicating that processing of the full-length phaseolin polypeptides was modified. Thus, although targeting of phaseolin to the protein body was not eliminated by removal of the glycan side-chains, decreased accumulation and stability of the full-length phaseolin protein in transgenic tobacco seed were evident.

Evolutionary conservation of transcriptional machinery between yeast and plants as shown by the efficient expression from the CaMV 35S promoter and 35S terminator

Complementation of fission yeast mutants by plant genomic libraries could be a promising method for the isolation of novel plant genes. One important prerequisite is the functioning of plant promoters and terminators in Schizosaccharomyces pombe and Saccharomyces cerevisiae. Therefore, we studied the expression of the bacterial beta-glucuronidase (GUS) reporter gene under the control of the Cauliflower Mosaic Virus (CaMV) 35S promoter and 35S terminator. We show here that S. pombe initiates transcription at exactly the same start site as was reported for tobacco. The 35S CaMV terminator is appropriately recognized leading to a polyadenylated mRNA of the same size as obtained in plant cells transformed with the same construct. Furthermore, the GUS-mRNA is translated into fully functional GUS protein, as determined by an enzymatic assay. Interestingly, expression of the 35S promoter in the budding yeast S. cerevisiae was found to be only moderate and about hundredfold lower than in S. pombe. To investigate whether different transcript stabilities are responsible for this enormous expression difference in the two yeasts, the 35S promoter was substituted by the ADH (alcohol dehydrogenase) promoter from fission yeast. In contrast to the differential expression pattern of the 35S promoter, the ADH promoter resulted in equally high expression rates in both fission and budding yeast, comparable to the 35S promoter in S. pombe. Since the copy number of the 35S-GUS constructs differs only by a factor of two in the two yeasts, it appears that differential recognition of the 35S promoter is responsible for the different transcription rates.

Expression of the beta-glucuronidase gene under the control of the CaMV 35s promoter in Schizosaccharomyces pombe

We have transformed Schizosaccharomyces pombe with the beta-glucuronidase (GUS) gene from Escherichia coli under the control of the plant cauliflower mosaic virus (CaMV) 35S promoter element. Efficient expression of GUS enzyme was observed. Moreover, transcription initiated at a unique site identical to that used in plant cells.

Expression, glycosylation and secretion of phaseolin in a baculovirus system

In this report, we describe the efficient expression and glycosylation, in insect cells, of β-phaseolin polypeptides (M r 45 and 48 kDa) from Phaseolus vulgaris, by means of a baculovirus expression vector. N-terminal sequence analysis demonstrated that the signal peptide was efficiently processed. Tunicamycin treatment suppressed both phaseolin bands seen in untreated or control cells, and resulted in a single species (M r 43 kDa). We provide evidence that the observed size heterogeneity arises by asymmetric glycosylation of a single, high-molecular weight precursor. These results also indicate that differential glycosylation of phaseolin polypeptides can occur on the product of a single gene, and, in that sense, is not dependent on amino acid sequence variations. Phaseolin accumulates to a very high level (90 µg/10(6) cells), 90% of it being secreted into the culture medium. Immuno-gold staining and electron microscopy demonstrated phaseolin polypeptides in electron-dense, membrane-bound vesicles seen at the periphery of the cytoplasm of infect cells and in cytoplasmic multivesicular bodies. The effect on protein accumulation of a single-basepair transversion (G»C) at position +6 is also described. This study constitutes, to our knowledge, one of the first instances of a plant protein being expressed in insect cells and suggests possible differences in the sorting mechanisms of glycoproteins from legume seeds and those from Spodoptera frugiperda cell line Sf9.

Plant intron sequences: evidence for distinct groups of introns

In vivo and in vitro RNA splicing experiments have demonstrated that the intron splicing machineries are not interchangeable in all organisms. These differences have prevented the efficient in vivo expression of monocot genes containing introns in dicot plants and the in vitro excision of some plant introns in HeLa cell in vitro splicing extracts. We have analyzed plant introns for sequence differences which potentially account for the functional splicing differences. Three classes of plant introns can be differentiated by the purine or pyrimidine-richness of sequences upstream from the 3' splice site. The frequency of these three types of introns in monocots and dicots varies significantly. The degree of variability in the 5' and 3' intron boundaries is evaluated for each of these classes in monocots and dicots. The 5' splice site consensus sequences developed for the monocot and dicot introns differ in their ability to base pair with conserved nucleotides present at the 5' end of many U1 snRNAs.

Isolation and expression of a pea vicilin cDNA in the yeast Saccharomyces cerevisiae

A cDNA clone containing the complete coding sequence for vicilin from pea (Pisum sativum L.) was isolated. It specifies a 50,000-Mr protein that in pea is neither post-translationally processed nor glycosylated. The cDNA clone was expressed in yeast from a 2 micron plasmid by using the yeast phosphoglycerate kinase promoter and initiator codon. The resultant fusion protein, which contains the first 16 amino acid residues of phosphoglycerate kinase in addition to the vicilin sequence, was purified and subsequently characterized. It has slightly slower mobility on SDS/polyacrylamide-gel electrophoresis than standard pea vicilin and forms a mixture of multimers, some of which resemble the native protein.

The plant vacuolar protein, phytohemagglutinin, is transported to the vacuole of transgenic yeast

Phytohemagglutinin (PHA), the major seed lectin of the common bean, Phaseolus vulgaris, accumulates in the parenchyma cells of the cotyledons. It has been previously shown that PHA is cotranslationally inserted into the endoplasmic reticulum with cleavage of the NH2-terminal signal peptide. Two N-linked oligosaccharide side chains are added, one of which is modified to a complex type in the Golgi apparatus. PHA is then deposited in membrane-bound protein storage vacuoles which are biochemically and functionally equivalent to the vacuoles of yeast cells and the lysosomes of animal cells. We wished to determine whether yeast cells would recognize the vacuolar sorting determinant of PHA and target the protein to the yeast vacuole. We have expressed the gene for leukoagglutinating PHA (PHA-L) in yeast under control of the yeast acid phosphatase (PHO5) promoter. Under control of this promoter, PHA-L accumulates to 0.1% of the total yeast protein. PHA-L produced in yeast is glycosylated as expected for a yeast vacuolar glycoprotein. Cell fractionation studies show that PHA-L is efficiently transported to the yeast vacuole. This is the first demonstration that vacuolar targeting information is recognized between two highly divergent species. A small proportion of yeast PHA-L is secreted which may be due to inefficient recognition of the vacuolar sorting signal because of the presence of an uncleaved signal peptide on a subset of the PHA-L polypeptides. This system can now be used to identify the vacuolar sorting determinant of a plant vacuolar protein.

Protein trafficking in plant cells

The cells of higher plants contain distinct subcellular compartments (organelles) that perform specialized functions such as photosynthesis, carbohydrate and lipid metabolism, and so forth. The majority of the protein constituents of plant organelles are formed as cytosolic precursors with N-terminal extensions that direct transport across one or more membrane bilayers in a post- or co-translational fashion. Since the majority of proteins in plant cells are products of nuclear gene expression, there must be precise sorting mechanisms in the cytoplasm that direct proteins to their correct cellular locations. Based on recent studies of protein targeting to chloroplasts and vacuoles, the details of these intracellular sorting mechanisms are becoming clear. The ability to direct proteins to specific compartments within cells provides new opportunities for improvement of plants by genetic manipulation.

Signal peptide specificity in posttranslational processing of the plant protein phaseolin in Saccharomyces cerevisiae

We linked the cDNA coding region for the bean storage protein phaseolin to the promoter and regulatory region of the Saccharomyces cerevisiae repressible acid phosphatase gene (PHO5) in multicopy expression plasmids. Yeast transformants containing these plasmids expressed phaseolin at levels up to 3% of the total soluble cellular protein. Phaseolin polypeptides in S. cerevisiae were glycosylated, and their molecular weights suggested that the signal peptide had been processed. We also constructed a series of plasmids in which the phaseolin signal-peptide-coding region was either removed or replaced with increasing amounts of the amino-terminal coding region for acid phosphatase. Phaseolin polypeptides with no signal peptide were not posttranslationally modified in S. cerevisiae. Partial or complete substitution of the phaseolin signal peptide with that from acid phosphatase dramatically inhibited both signal peptide processing and glycosylation, suggesting that some specific feature of the phaseolin signal amino acid sequence was required for these modifications to occur. Larger hybrid proteins that included approximately one-half of the acid phosphatase sequence linked to the amino terminus of the mature phaseolin polypeptide did undergo proteolytic processing and glycosylation. However, these polypeptides were cleaved at several sites that are not normally used in the unaltered acid phosphatase protein.

Signal peptide specificity in posttranslational processing of the plant protein phaseolin in Saccharomyces cerevisiae

We linked the cDNA coding region for the bean storage protein phaseolin to the promoter and regulatory region of the Saccharomyces cerevisiae repressible acid phosphatase gene (PHO5) in multicopy expression plasmids. Yeast transformants containing these plasmids expressed phaseolin at levels up to 3% of the total soluble cellular protein. Phaseolin polypeptides in S. cerevisiae were glycosylated, and their molecular weights suggested that the signal peptide had been processed. We also constructed a series of plasmids in which the phaseolin signal-peptide-coding region was either removed or replaced with increasing amounts of the amino-terminal coding region for acid phosphatase. Phaseolin polypeptides with no signal peptide were not posttranslationally modified in S. cerevisiae. Partial or complete substitution of the phaseolin signal peptide with that from acid phosphatase dramatically inhibited both signal peptide processing and glycosylation, suggesting that some specific feature of the phaseolin signal amino acid sequence was required for these modifications to occur. Larger hybrid proteins that included approximately one-half of the acid phosphatase sequence linked to the amino terminus of the mature phaseolin polypeptide did undergo proteolytic processing and glycosylation. However, these polypeptides were cleaved at several sites that are not normally used in the unaltered acid phosphatase protein.

Expression of the RNA genome of an animal virus in Saccharomyces cerevisiae

The nucleocapsid of vesicular stomatitis virus (VSV) was introduced into the cytoplasm of Saccharomyces cerevisiae by low pH-dependent fusion of the viral envelope with the spheroplast plasma membrane. This led to de novo synthesis of the three major structural proteins of the virus--the G, N, and M proteins--as shown by immunoprecipitation of [35S]methionine-labeled spheroplast lysates. In NaDodSO4/polyacrylamide gel electrophoresis, M and N proteins comigrated with those of the virion, whereas the yeast-made G protein migrated as two bands with apparent molecular sizes of 60 and 70 kDa. Both polypeptides appeared to be N-glycosylated, since only one polypeptide with the apparent molecular mass of approximately equal to 55 kDa was produced in the presence of tunicamycin. Phase separation into Triton X-114 suggested that the unglycosylated G protein was membrane bound. According to immunofluorescent surface staining of live spheroplasts, at least part of the G protein was transported to the plasma membrane. Spheroplasts expressing the VSV genes could be fused together by low pH to form polykaryons, indicating that G protein synthetized by yeast was fusogenic--i.e., biologically active.

The expression of a nopaline synthase - human growth hormone chimaeric gene in transformed tobacco and sunflower callus tissue

To study whether mammalian RNA processing signals function in plants, we have constructed a chimaeric gene in which the complete human growth hormone (hGH) gene is flanked by DNA fragments containing the promoter and polyadenylation site of the nopaline synthase gene. The hGH gene used contains four introns and an additional 440 bp downstream from the hGH poly(A) addition site. The transcription of this chimaeric gene was studied following its introduction into sunflower and tobacco cells using a Ti plasmid vector. Analysis of poly(A)(+) RNA isolated from the transformed tumor tissue demonstrated the following: (1) a single polyadenylated transcript, 2700 bp in length, was transcribed from the chimaeric gene; (2) the transcription was initiated at the published start site of the nopaline synthase gene; (3) the hGH polyadenylation site was not used for processing of the 3' end; only the poly(A) addition site of the nopaline synthase gene was recognized, (4) no splicing of the hGH introns could be detected. We also demonstrate that the hGH pre-mRNA isolated from plant cells can be spliced in a HeLa cell nuclear extract, indicating that the hGH pre-mRNA was functional. These results show that processing signals of the hGH pre-mRNA are not recognized in these plant cells.