BBA - Reviews on Biomembranes 737 (1983) 267-284. doi:10.1016/0304-4157(83)90003-5Received by publisher: 1982-10-06Harvest Date: 2016-01-04 12:22:08DOI: 10.1016/0304-4157(83)90003-5Page Range: 267-28

Zeins, the storage proteins of maize, are totally lacking in the essential amino acids lysine and tryptophan. Lysine codons and lysine- and tryptophan-encoding oligonucleotides were introduced at several positions into a 19-kilodalton zein complementary DNA by oligonucleotide-mediated mutagenesis. A 450-base pair open reading frame from a simian virus 40 (SV40) coat protein was also engineered into the zein coding region. Messenger RNAs for the modified zeins were synthesized in vitro with an SP6 RNA polymerase system and injected into Xenopus laevis oocytes. The modifications did not affect the translation, signal peptide cleavage, or stability of the zeins. The ability of the modified zeins to assemble into structures similar to maize protein bodies was assayed by two criteria: assembly into membrane-bound vesicles resistant to exogenously added protease, and ability to self-aggregate into dense structures. All of the modified zeins were membrane-bound; only the one containing a 17-kilodalton SV40 protein fragment was unable to aggregate. These findings suggest that it may be possible to create high-lysine corn by genetic engineering.

We have isolated full-length cDNA clones that encode oat (Avena sativa) seed storage globulin mRNAs from a cDNA library in the expression vector lambda gtll. The longest of these clones, pOG2, has an 1840-base pair insert that encodes a complete precursor subunit with a signal peptide of 24 amino acids followed by an acidic polypeptide of 293 amino acids and a basic polypeptide of 201 amino acids. Near the C terminus of the acidic polypeptide are four repeats of a higly conserved, glutamine-rich octapeptide. Other oat globulin cDNA clones contain five of these repeats. Nucleotide sequence comparisons between these clones indicate that the genes encoding these proteins are highly conserved. We estimate there to be 7 to 10 genes for the oat globulin per haploid genome. Comparisons of amino acid sequences show that the oat globulin is 30 to 40% homologous with storage globulins of legumes and about 70% homologous with the rice seed storage globulin (glutelin).During their development, plant seeds accumulate large amounts of storage proteins that serve as sources of nitrogen, sulfur, and carbon compounds during seed germination (25). Two major classes of storage proteins can be distinguished: globulins, which are found principally in the cotyledons and axis of the embryo, and prolamines, which are found in the endosperm of cereal seeds (13). Two major types of storage globulins have been described that have sedimentation coefficients ofabout 7S and 11S. The proportion of the two types of globulins is variable among dicots; monocot embryos contain only the 7S globulin, which is present in the scutellum.Storage globulins account for most of the protein in dicot seeds, but they generally make up only a small fraction of the protein found in cereal seeds. Instead, most cereals contain predominantly prolamine-type storage proteins. Oats and rice are exceptions. These two contain only small amounts ofprolamine (5-10%), and most of their storage protein is an 11 to 12S globulin that is synthesized in the endosperm. Both of these proteins are structurally related to the 11S globulins found in dicots (25), but both are much less soluble than the dicot 11S globulins. The oat globulin requires 0.8 to 1.0 M NaCl for solubility, whereas the characteristics of the rice globulin (glutelin) are such that denaturing solvents are required for solvation.Previous studies in our laboratory (29) and elsewhere (5, 7)

We have isolated and characterized cDNA clones encoding avenins, the prolamine storage proteins of oat seeds. Sequence analysis shows that avenins are a related group of polypeptides and that their mRNAs differ from each other by point mutations and small insertions and deletions. Avenin proteins have structural homology to the a/@-gliadins and r-gliadins of wheat, the B-hordeins of barley, and the r-secalins of rye. Hybridization analysis of DNA from various diploid, tetraploid, and hexaploid oat species shows that the oat genome contains more globulin storage protein genes than avenin genes and that some restriction fragments containing these genes are conserved between species with common genomes. We estimate that there are 25 avenin genes and 50 globulin genes per haploid genome in Avena sativa and similar ratios of globulin to avenin genes in other Avena species. Avenin and globulin polypeptides begin to accumulate between 4 days and 6 days after anthesis. Messenger RNAs encoding avenin and globulin proteins become abundant 4 days after anthesis and reach peak concentrations at 8 days after anthesis. Avenin mRNAs are present in somewhat greater molar amounts than globulin mRNAs beginning at 4 days after anthesis. Because there is considerably more globulin than avenin in the mature oat seed, the expression of globulin and avenin genes may be regulated both transcriptionally and post-transcriptionally.

Animal cells regulate the activities of neutral amino acid transport Systems A and L to keep the intracellular supply of amino acids relatively constant. Transport System A activity increases dramatically in response to starvation of all the amino acids. Transport System L activity increases in response to starvation of a single substrate such as leucine. The mechanism of regulation appears to be different for Systems A and L.