Upon seed germination, this peptide is degraded, suggesting that it plays an important role in the storage of nitrogen andor sulfur.Wiles Find this author on Google Scholar Find this author on PubMed Search for this author on this site Joshua S.
Sharp Find this author on Google Scholar Find this author on PubMed Search for this author on this site Fred R. Naider Find this author on Google Scholar Find this author on PubMed Search for this author on this site Jeffrey M. Becker Find this author on Google Scholar Find this author on PubMed Search for this author on this site Gary Stacey Find this author on Google Scholar Find this author on PubMed Search for this author on this site. These proteins show significant sequence similarity to OPTs of Candida albicans (CaOpt1p), Schizosaccharomyces pombe (Isp4p), and Saccharomyces cerevisiae (Opt1p and Opt2p). Hydrophilicity plots of the OPTs suggest that they are integral membrane proteins with 12 to 14 transmembrane domains. Sequence comparisons showed that the AtOPTs form a distinct subfamily when compared with the fungal OPTs. Two highly conserved motifs (NPG and KIPPR) were found among all OPT members. The identification of multiple OPTs in Arabidopsis suggests that they may play different functional roles. This idea is supported by the fact that AtOPT s have a distinct, tissue-specific expression pattern. The cDNAs encoding seven of the AtOPTs were cloned into a yeast vector under the control of a constitutive promoter. Similarly, expression of five of the seven AtOPT proteins expressed in yeast conferred the ability to uptake tetra- and pentapeptides as measured by growth. This study provides new evidence for multiple peptide transporter systems in Arabidopsis, suggesting an important physiological role for small peptides in plants. After uptake, the internalized peptides are rapidly hydrolyzed by peptidases and used as a source of amino acids, nitrogen, or carbon. Compared with prokaryotes and animals, peptide transport in higher plants has received little attention. There are few published reports dealing with small peptides in plants. Higgins and Payne (1982) reported that significant levels of peptides were found in phloem and xylem exudates. These included the non-protein-derived peptides (e.g. The xylem contains lower levels of nitrogen than the phloem, yet peptides were reported in the xylem exudates of several species, such as the vegetative organs and berries of grape ( Khachidze, 1975 ) and the sap of corn ( Fejer and Konay, 1958 ). Higgins and Payne (1982) suggested that the transport of peptides is a more efficient means of nitrogen distribution than the transport of individual amino acids. This would be especially true for long-distance transport during the bulk movement of protein-degradation products (e.g. As nitrogen carriers, peptides may also protect amino acids from catabolism by enzymes known to be present in the phloem during transport within the plant ( Higgins and Payne, 1980 ). Glutathione (GSH), a modified tripeptide (-Glu-Cys-Gly), has also been suggested as a carrier of reduced sulfur in the phloem and xylem ( Higgins and Payne, 1982 ). ![]() For example, 34 of the non-protein amino nitrogen of a kidney bean seed is present as -glutamyl- S -methyl- l -Cys ( Goore and Thompson, 1967 ).
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