SIEVE-TUBE EXUDATE FROM RICINUS-COMMUNIS L SEEDLINGS CONTAINS UBIQUITIN AND CHAPERONES

The cut hypocotyl of Ricinus communis L. seedlings exudes phloem sap w hich contains a characteristic set of proteins (Sakuth et al. 1993, Pl anta 191, 207-213). These sieve-tube exudate proteins were probed with antibodies to highly conserved proteins, namely ribulose-1,5-bisphosp hate carboxylase-oxygenase (Rubisco), Rubisco-subunit-binding protein, heat-shock protein (HSP 70), chaperonin GroEL and ubiquitin. Homologo us proteins in the sieve-tube exudate were identified with antisera to HSP 70, Rubisco-subunit-binding protein and ubiquitin. Ribulose-1,5-b isphosphate carboxylase-oxygenase, which was present in the tissue, wa s 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 c otyledonary tissues. Therefore, ubiquitin features properties which fa vour its transfer into the sieve tubes and which might rely on efficie nt 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 p rotein translocation through plasmodesmata from companion cells to sie ve tubes is discussed,