Characterization of Ricinus communis phloem profilin, RcPRO1

The mature, functional sieve tube, which forms the conduit for assimilate d istribution in higher plants, is dependent upon protein import from the com panion cells for maintenance of the phloem long-distance translocation syst em. Using antibodies raised against proteins present in the sieve-tube exud ate of Ricinus communis (castor bean) seedlings, a cDNA was cloned which en coded a putative profilin, termed RcPRO1. Expression and localization studi es indicated that RcPRO1 mRNA encodes a phloem profilin, with some expressi on occurring in epidermal, cortex, pith and xylem tissue. Purified, recombi nant RcPRO1 was functionally equivalent to recombinant maize profilin ZmPRO 4 in a live cell nuclear displacement assay. The apparent equilibrium disso ciation constant for RcPRO1 binding to plant monomeric (G-)actin was lower than the previously characterized maize profilins. Moreover, the affinity o f RcPRO1 for poly-L-proline (PLP) was significantly higher than that for re combinant maize profilins. Within the sieve-tube exudate, profilin was pres ent in 15-fold molar excess to actin. The data suggest that actin filament formation is prevented within the assimilate stream. These results are disc ussed in terms of the unique physiology of the phloem.