Schobert, C; Grossmann, P; Gottschalk, M; Komor, E; Pecsvaradi, A; Zurnieden, U: Sieve-tube exudate from ricinus-communis l seedlings contains ubiquitin and chaperones, Planta, 196, 205-210 (1995)
The cut hypocotyl of Ricinus communis L. seedlings exudes phloem sap which contains a characteristic set of proteins (Sakuth et al. 1993, Planta 191, 207-213). These sieve-tube exudate proteins were probed with antibodies to highly conserved proteins, namely ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), Rubisco-subunit-binding protein, heat-shock protein (HSP 70), chaperonin GroEL and ubiquitin. Homologous proteins in the sieve-tube exudate were identified with antisera to HSP 70, Rubisco-subunit-binding protein and ubiquitin. Ribulose-1,5-bisphosphate carboxylase-oxygenase, which was present in the tissue, was not detected. Of all the cross-reactive proteins detected, ubiquitin was special because the ubiquitin-to-protein ratio in the sieve-tube exudate was higher than in both the surrounding hypocotyl and in the cotyledonary tissues. Therefore, ubiquitin features properties which favour its transfer into the sieve tubes and which might rely on efficient transport through plasmodesmata. It is assumed that chaperones and ubiquitin are needed for the maintenance of sieve-tube function, e.g. to ensure correct folding of proteins. Their possible involvement in protein translocation through plasmodesmata from companion cells to sieve tubes is discussed,

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