Studies on the peptide transport system in the scutellum of germinating barley
Through the use of protein-modification reagents, in particular the thiol-specific reagents N-ethylmaleimide, p -chloromercuribenzenesulphonic acid, and phenylarsine oxide, it is shown that in the barley scutellum, the transport of peptides, but not the transport of amino acids or glucose, is specifically thiol-dependent. Furthermore, these essential thiol groups are shown to exist as redox-sensitive, vicinal dithiols which lie at the substrate-binding sites of the peptide transport proteins. A technique for the specific labelling of the vicinal dithiols with radioactive N-ethylmaleimide is described, and microautoradiography of scutellar tissue treated in this way shows these groupings to be mainly located in the plasmaleramae of the scutellar epithelium. In related work, the importance of arginyl and histidyl residues to both peptide and amino acid transport is shown, although other moieties, e.g., carboxyl ligands, would not seem to be critically involved. In other studies concerning the general characteristics of these transport systems in barley, the development of peptide and amino acid transport capacity during germination is described, and it seems that initially, peptide transport is likely to be the more important in the nitrogen nutrition of the embryo. A proton-motive force is implicated in the energisation of transport by the scutellum, and some evidence was obtained to indicate that a proton gradient was necessary to maintain the essential dithiol groups in their functional state.