LAUSR

Rubber elongation factor (REF) and small rubber particle protein (SRPP) are major latex proteins harvested from Hevea brasiliensis (the rubber tree; HbREF and HbSRPP, respectively). Their amino acid sequences exhibit high homology with each other. In the present study, we cloned two cDNAs encoding REF/SRPP-family proteins (FcREF/SRPP-1 and -2) from the laticifers of Ficus carica (fig tree). The amino acid sequences of these proteins showed high homology not only with each other but also with HbREF and HbSRPP. Recombinant FcREF/SRPP-1 and -2 were expressed in E. coli, and their aggregation properties were examined using a Congo red binding assay, agarose gel electrophoresis, and transmission electron microscopy. FcREF/SRPP-1 formed fibrils when incubated in PBS, and grew to micrometer-sized amorphous aggregates that precipitated rapidly. These aggregation properties of FcREF/SRPP-1 are quite similar to those of HbREF, although the growth rate and size of FcREF/SRPP-1 aggregates were inferior to those of HbREF. FcREF/SRPP-2 also formed aggregates during the incubation, but they did not precipitate, as has been reported for HbSRPP. Our results suggest that FcREF/SRPP-1 and -2 correspond to HbREF and HbSRPP, respectively. These aggregation properties could provide useful benchmarks for classifying REF/SRPP-family proteins as REF or SRPP.