Nucleotide sequence analysis of alpha-amylase and thiol protease genes that are hormonally regulated in barley aleurone cells

Identification of an endochitinase cDNA clone from barley aleurone cells

A barley aleurone cDNA library was screened using (32)P-labeled cDNA prepared by reverse transcription of mRNA from aleurone layers treated in the presence or absence of gibberellic acid (GA). Besides α-amylase cDNA clones, another set of clones representing an abundant mRNA in aleurone cells was identified. Messenger RNA hybrid-selected by a prototype clone of this group (clone 10) was translated in vitro to yield a 36 kDa protein. Analysis of the DNA sequence and the predicted amino acid sequence of the protein product of this clone indicates that this gene codes for a protein with homology to endochitinases from tobacco and bean. In addition, the predicted amino acid sequence includes a stretch that is closely related to a cyanogen bromide cleavage fragment from an endochitinase isolated from barley endosperm. The structural genes for endochitinase are present as multiple copies on barley chromosome 1. mRNA detectable by this clone increases in abundance in barley aleurone cells incubated in the absence and in the presence of GA. Western blot analysis of proteins from aleurone and endosperm tissues indicates the presence of multiple endochitinases differing in molecular size. Possible mechanisms for the occurrence of multiple forms of endochitinases and their biological significance are discussed.

Protein secretion in plants

Protein secretion is an ubiquitous but poorly understood process in plants. Secreted proteins are synthesized on the membranes of the rough endoplasmic reticulum and transported to the cell surface by secretary vesicles formed at the Golgi apparatus. Whereas many of the structural details of this process are known the mechanisms underlying secretion are just beginning to be understood, in this article we review some of the recent developments in this field, and we compare the progress made with animal and plant cells. CONTENTS Summary 567 I. Introduction 568 II. Proteins secreted by plants 568 III. Synthesis and post-translational modification of secreted proteins 571 IV. Molecular requirements for secretion 576 V. Vehicles of secretory transport 581 VI. Regulation of secretion 585 VII. Conclusions and Perspective 587 Acknowledgements 588 References 588.

The 5' splice site: phylogenetic evolution and variable geometry of association with U1RNA

The 5' splice site sequences of 3294 introns from various organisms (1-672) were analyzed in order to determine the rules governing evolution of this sequence, which may shed light on the mechanism of cleavage at the exon-intron junction. The data indicate that, currently, in all organisms, a common sequence 1GUAAG6U and its derivatives are used as well as an additional sequence and its derivatives, which differ in metazoa (G/1GUgAG6U), lower eucaryotes (1GUAxG6U) and higher plants (AG/1GU3A). They all partly resemble the prototype sequence AG/1GUAAG6U whose 8 contigous nucleotides are complementary to the nucleotides 4-11 of U1RNA, which are perfectly conserved in the course of phylogenetic evolution. Detailed examination of the data shows that U1RNA can recognize different parts of 5' splice sites. As a rule, either prototype nucleotides at position -2 and -1 or at positions 4, 5 or 6 or at positions 3-4 are dispensable provided that the stability of the U1RNA-5' splice site hybrid is conserved. On the basis of frequency of sequences, the optimal size of the hybridizable region is 5-7 nucleotides. Thus, the cleavage at the exon-intron junction seems to imply, first, that the 5' splice site is recognized by U1RNA according to a "variable geometry" program; second, that the precise cleavage site is determined by the conserved sequence of U1RNA since it occurs exactly opposite to the junction between nucleotides C9 and C10 of U1RNA. The variable geometry of the U1RNA-5' splice site association provides flexibility to the system and allows diversification in the course of phylogenetic evolution.

Sequence heterogeneity and differential expression of the alpha-Amy2 gene family in wheat

The alpha-Amy2 genes of wheat are a multigene family which is expressed in the aleurone cells of germinating grain under control of the plant hormone gibberellin. A subset of the genes are also expressed in developing grain. Comparison of five genomic clones containing alpha-Amy2 genes, using DNA sequence analysis and Southern hybridisation, showed that the extent of similarity between genes differed. Two of the most heterogeneous genes compared were located to the same group 7 chromosome while the most similar genes alpha-Amy2/54 and alpha-Amy2/8 were located to different ones; hence sequence variation could not be correlated to the ancestry of the alpha-Amy2 genes during the separate existence of the constituent genomes of hexaploid wheat. Expression of the cloned genes was measured using an S1 nuclease protection assay and this identified alpha-Amy2/54 and alpha-Amy2/8 as part of the subset of alpha-Amy2 genes expressed in both the developing grain and in aleurone cells. Comparison of the 5' upstream regions of all five genes showed high similarity, with the exception of one gene, up to -280 nucleotides from the transcriptional start, while similarity between alpha-Amy2/54 and alpha-Amy2/8 extended a further 90 bp upstream of this point. It is suggested that regulatory elements responsible for tissue specificity and gibberellin regulation may be located within these regions of similarity.

Molecular analysis of actinidin, the cysteine proteinase of Actinidia chinensis

We have isolated and sequenced two very similar cDNA clones of 1145 and 809 bp length, from a fruit-specific library of Actinidia chinensis, the larger encoding all 220 amino acids of actinidin, showing 91% homology to the published amino acid sequence. Both cDNAs code for an additional 25 amino acids following the mature carboxy terminus of actinidin. The larger clone has coding potential for 57 residues of an amino-terminal extension with considerable homology to amino-terminal sequences of other cysteine proteinases. From size determination of both mRNA (1.4 kb) and immunoprecipitated in vitro translation product (39 kDa) it was estimated that actinidin is synthesised as a precursor approximately 15 kDa larger than the mature protein. Both proteolytic cleavage sites are located on the surface of the molecule as illustrated by the hydropathy profile of the deduced amino acid sequence. Features of the prosegment primary sequence are considered with regard to a possible mechanism of inactivation of the proteinase, by analogy with other proteolytic zymogens. The presence of three potential glycosylation sites, one within the carboxy-terminal and two in the amino-terminal extension, are consistent with subcellular location of the enzyme within membrane-bound organelles. Results from a Southern blot suggest that actinidin is encoded by a multigene family of up to ten members. Actinidin gene expression, both at the level of mRNA and protein, is largely restricted to the fruit of the plant, where the level of actinidin mRNA accumulates early during development.

Intron conservation across the prokaryote-eukaryote boundary: structure of the nuclear gene for chloroplast glyceraldehyde-3-phosphate dehydrogenase from maize

The nuclear gene encoding chloroplast glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from maize has been cloned and sequenced. The gene is G + C rich in its coding sequences and, in addition, contains a CpG-rich region surrounding the promoter. Further upstream several enhancer-like repetitions have been identified that may control the light- and phytochrome-mediated expression of this gene. The gene is interrupted by three introns. Introns 1 and 2 are located within the sequence encoding the transit peptide, dividing it into three parts, each containing one of the three major homology blocks typical for transit peptides of nucleus-encoded chloroplast proteins. Intron 3 is located at codon 166 (glycine) at the same nucleotide position as intron 1 in the GAPDH gene from the nematode Caenorhabditis elegans, suggesting that this intron was present in the parental GAPDH gene from which these two modern descendants originated. Intron 3 divides the GAPDH protein into its two constituent domains, the NAD-binding and the catalytic domain, immediately after helix alpha 1 at a position homologous to that of intron 9 in the gene for maize alcohol dehydrogenase, thereby confirming the prediction of Brändén et al. on the basis of gene-protein structure correlations in maize alcohol dehydrogenase for the placement of introns in the GAPDH gene [Brändén, C.I., Eklund, H., Cambillau, C. & Pryor, A.J. (1984) EMBO J. 3, 1307-1310]. These results suggest that intron 3 is an archetypical relic of early GAPDH and alcohol dehydrogenase evolution, whereas introns 1 and 2 were implicated in the evolution of chloroplast transit peptides.

RNA complementary to α-amylase mRNA in barley

Two experimental approaches demonstrate that different types of RNA complementary to α-amylase mRNA are present in barley. S1 nuclease assays identify an RNA that is complementary to essentially the full length of both the type A and type B α-amylase mRNAs. Complementarity, however, is imperfect: the S1 nuclease-resistant products can only be identified if they are electrophoresed as RNA-DNA hybrids. This RNA is present in developing endosperm + aleurone tissue and in mature aleurone tissue cultured in the absence of hormonal treatment or in the presence of abscisic acid, but not in shoot or root tissue. In mature aleurone tissue treated with abscisic acid, its steady-state abundance is similar to that of α-amylase mRNA. Northern blot analysis indicated the presence of a second type of antisense RNA. Under conditions of moderate stringency, antisense-specific probes detect discrete hybridizing species of 1.6, 1.4, and 1.0 kilobases in mature aleurone and shoot tissues that do not represent spurious "hybridization" to rRNA, α-amylase mRNA, or the abundant, G+C-rich mRNA for a probable amylase/protease inhibitor. The different results are consistent with the fact that the hybridization assay can tolerate relatively short regions of complementarity separated by large, nonhomologous sequences, while the nuclease protection assay cannot.

Regulation of activity of calcium activated neutral protease

Various lines of evidence suggest that calcium dependent protease (CANP, calpain) exists in the cytosol as an inactive proenzyme which is converted to an active form by autolysis. During autolysis only the N-terminal regions of both subunits of proCANP are modified. About 20 and 90 residues are removed from the large and small subunits, respectively. The N-terminal region (domain I) of the large subunit modified during autolysis precedes the protease domain and corresponds to the propeptides of various cysteine proteinases. Analyses of the autocatalytic activation of CANP in the presence of plasma membranes reveal that proCANP translocates to the membrane in the presence of microM Ca2+ and is activated at the membrane. The CANP inhibitor and Ca2+ are the most important factors for the regulation of CANP activity. The primary translation product of the mRNA for rabbit liver CANP inhibitor contains four internal repeats. Structural analyses of the liver and erythrocyte inhibitors reveal that they contain four and three repeats, respectively. The repeating unit was identified as the functional unit of the inhibitor and each unit inhibits one mole of CANP. On the basis of these results, an activation mechanism for proCANP at the membrane was proposed. The native enzyme, which has been called CANP or calpain, should now be called proCANP or calpainogen. CANP and calpain should be used for the autolyzed active form.