Zein synthesis in maize endosperm by polyribosomes attached to protein bodies.
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The protein bodies in maize endosperm are the sites of zein deposition. They are single membrane-bound vesicles with polysomes associated with the exterior surface of the membrane. These protein bodies were isolated by sucrose density gradients and characterized by electron microscopy and polyacrylamide gel electrophoresis. Polyribosomes dissociated from the surface of the membrane by detergent treatment were placed into an amino-acid incorporating system. Based on alcohol solubility, amino-acid composition, and molecular weight distribution, the product synthesized appeared to be largely, or entirely, zein. This suggests the existence of components which are specific for the synthesis of zein at the protein body membrane surface. (+info)
Specific combinations of zein genes and genetic backgrounds influence the transcription of the heavy-chain zein genes in maize opaque-2 endosperms.
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The transcript levels of heavy-chain zein genes (zH1 and zH2) and the occurrence of the zH polypeptides in different opaque-2 (o2) lines were investigated by RNA-blot analyses and by sodium dodecylsulfate-polyacrylamide gel electrophoresis or two-dimensional gel electrophoresis protein fractionations. Four mutant alleles o2R, o2T, o2It, and o2-676 introgressed into different genetic backgrounds (GBs) were considered. The mono-dimensional gel electrophoresis zein pattern can be either conserved or different among the various GBs carrying the same o2 allele. Likewise, in the identical GB carrying different o2 alleles, the zein pattern can be either conserved or differentially affected by the different mutant allele. Zein protein analysis of reciprocal crosses between lines with different o2 alleles or the same o2 showed in some case a more than additive zH pattern in respect to the o2 parent lines. Electrophoretic mobility shift assay approaches, with O2-binding oligonucleotide and endosperm extracts from the above o2 lines, failed to reveal o2-specific retarded band in any of the o2 extracts. The results suggest that the promoter of some zH1 and zH2 contains motif(s) that can respond to factors other than O2. (+info)
Identification of free fatty acids in maize protein bodies and purified alpha zeins by (13)C and (1)H nuclear magnetic resonance.
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Zeins, the maize storage proteins, are the most abundant proteins in the corn endosperm, and are synthesized on the rough endoplasmatic reticulum and deposited in discrete organelles called protein bodies. Several authors, using circular dichroism and optical rotatory dispersion, have concluded that these proteins have a high alpha-helical content in alcoholic solution. In this work we have studied these proteins, within the protein bodies themselves and after extraction from the corn grains with 70% ethanol, using NMR (nuclear magnetic resonance) spectroscopy. We conclusively demonstrate the presence of free fatty acids within both the protein bodies and also in the alcohol extracted alpha zeins. We present evidence for a direct interaction between the free fatty acids and the alpha zein proteins within the protein body and suggest possible mechanisms by which such an association has arisen during the evolution of the maize endosperm. (+info)
Free amino acids in crocodilians fed proteins of different biological value.
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Changes in plasma levels of amino acids derived from fed protein were determined by feeding crocodilians (Caiman crocodilus crocodilus and Alligator mississipiensis) 7.5 g protein/kg body weight and by monitoring the plasma free amino acids for several days. Zein and several other vegetable proteins produced no rise in plasma amino acids and were excreted intact in the feces. Casein and fish muscle were rapidly digested but produced little rise in plasma amino acids, and the increases showed no relationship to the composition of the protein fed. Gelatin feeding led to large increases in plasma amino acids that persisted for more than a week, and the resulting pattern was nearly identical to the composition of gelatin with the exception of aspartic and glutamic acids, and several animals died. Equivalent quantities of fish muscle protein were assimilated without difficulty by the crocodilians. Endogenous protein secreted into the gut apparently contributed little to the amino acid mixture absorbed. (+info)
Genomics analysis of genes expressed in maize endosperm identifies novel seed proteins and clarifies patterns of zein gene expression.
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We analyzed cDNA libraries from developing endosperm of the B73 maize inbred line to evaluate the expression of storage protein genes. This study showed that zeins are by far the most highly expressed genes in the endosperm, but we found an inverse relationship between the number of zein genes and the relative amount of specific mRNAs. Although alpha-zeins are encoded by large multigene families, only a few of these genes are transcribed at high or detectable levels. In contrast, relatively small gene families encode the gamma- and delta-zeins, and members of these gene families, especially the gamma-zeins, are highly expressed. Knowledge of expressed storage protein genes allowed the development of DNA and antibody probes that distinguish between closely related gene family members. Using in situ hybridization, we found differences in the temporal and spatial expression of the alpha-, gamma-, and delta-zein gene families, which provides evidence that gamma-zeins are synthesized throughout the endosperm before alpha- and delta-zeins. This observation is consistent with earlier studies that suggested that gamma-zeins play an important role in prolamin protein body assembly. Analysis of endosperm cDNAs also revealed several previously unidentified proteins, including a 50-kD gamma-zein, an 18-kD alpha-globulin, and a legumin-related protein. Immunolocalization of the 50-kD gamma-zein showed this protein to be located at the surface of prolamin-containing protein bodies, similar to other gamma-zeins. The 18-kD alpha-globulin, however, is deposited in novel, vacuole-like organelles that were not described previously in maize endosperm. (+info)
Utilization of nitrogen from soybean meal, casein, zein, and urea by mature sheep.
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The utilization of nitrogen contained in soybean meal, casein, zein, and urea was studied in 12 mature wethers. Net microbial synthesis rates during the 6-hour period after feeding corresponded to ruminal ammonia concentrations, suggesting that the ruminal ammonia level in these experiments was a limiting factor in microbiol protein synthesis. Maximum conversion of dietary nitrogen to microbial nitrogen in the rumen during a 24-hour period was estimated to be 62.5, 85.3, 40.9, and 90.1%, respectively, for soybean meal, casein, zein, and urea. Estimated production rates of acetic, propionic, butyric, isovaleric, and valeric acids during the first 6 hours after feeding suggested that zein supported the lowest microbial activity in the rumen, and that urea supplementation resulted in isovaleric and valeric acids production rates equivalent to or greater than rates when the other nitrogen supplements were present in the diet. Daily endogenous urinary and metabolic fecal nitrogen determined by regression analysis utilizing soybean meal as the only nitrogen supplement were 72 mg/kg body weight0.75 and 340 mg/100 g of dry matter intake. Biological values of 84.8, 78.3, 84.3, and 76.4 were obtained for soybean meal, casein, zein, and urea, respectively. (+info)
Sequence, regulation, and evolution of the maize 22-kD alpha zein gene family.
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We have isolated and sequenced all 23 members of the 22-kD alpha zein (z1C) gene family of maize. This is one of the largest plant gene families that has been sequenced from a single genetic background and includes the largest contiguous genomic DNA from maize with 346,292 bp to date. Twenty-two of the z1C members are found in a roughly tandem array on chromosome 4S forming a dense gene cluster 168,489-bp long. The twenty-third copy of the gene family is also located on chromosome 4S at a site approximately 20 cM closer to the centromere and appears to be the wild-type allele of the floury-2 (fl2) mutation. On the basis of an analysis of maize cDNA databases, only seven of these genes appear to be expressed including the fl2 allele. The expressed genes in the cluster are interspersed with nonexpressed genes. Interestingly, some of the expressed genes differ in their transcriptional regulation. Gene amplification appears to be in blocks of genes explaining the rapid and compact expansion of the cluster during the evolution of maize. (+info)
Zein protein interactions, rather than the asymmetric distribution of zein mRNAs on endoplasmic reticulum membranes, influence protein body formation in maize endosperm.
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Prolamin-containing protein bodies in maize endosperm are composed of four different polypeptides, the alpha-, beta-, gamma-, and delta-zeins. The spatial organization of zeins within the protein body, as well as interactions between them, suggests that the localized synthesis of gamma-zeins could initiate and target protein body formation at specific regions of the rough endoplasmic reticulum. To investigate this possibility, we analyzed the distribution of mRNAs encoding the 22-kD alpha-zein and the 27-kD gamma-zein proteins on cisternal and protein body rough endoplasmic reticulum membranes. In situ hybridization revealed similar frequencies of the mRNAs in both regions of the endoplasmic reticulum, indicating that the transcripts are distributed more or less randomly. This finding implies that zein protein interactions determine protein body assembly. To address this question, we expressed cDNAs encoding alpha-, beta-, gamma-, and delta-zeins in the yeast two-hybrid system. We found strong interactions among the 50-, 27-, and 16-kD gamma-zeins and the 15-kD beta-zein, consistent with their colocalization in developing protein bodies. Interactions between the 19- and 22-kD alpha-zeins were relatively weak, although each of them interacted strongly with the 10-kD delta-zein. Strong interactions were detected between the alpha- and delta-zeins and the 16-kD gamma-zein and the 15-kD beta-zein; however, the 50- and 27-kD gamma-zeins did not interact with the alpha- and delta-zein proteins. We identified domains within the 22-kD alpha-zein that bound preferentially the alpha- and delta-zeins and the beta- and gamma-zeins. Affinities between zeins generally were consistent with results from immunolocalization experiments, suggesting an important role for the 16-kD gamma-zein and the 15-kD beta-zein in the binding and assembly of alpha-zeins within the protein body. (+info)