Rubber Elongation Factor (REF), a Major Allergen Component in Hevea brasiliensis Latex Has Amyloid Properties

Abstract

REF (Hevb1) and SRPP (Hevb3) are two major components of Hevea brasiliensis latex, well known for their allergenic properties. They are obviously taking part in the biosynthesis of natural rubber, but their exact function is still unclear. They could be involved in defense/stress mechanisms after tapping or directly acting on the isoprenoid biosynthetic pathway. The structure of these two proteins is still not described. In this work, it was discovered that REF has amyloid properties, contrary to SRPP. We investigated their structure by CD, TEM, ATR-FTIR and WAXS and neatly showed the presence of β-sheet organized aggregates for REF, whereas SRPP mainly fold as a helical protein. Both proteins are highly hydrophobic but differ in their interaction with lipid monolayers used to mimic the monomembrane surrounding the rubber particles. Ellipsometry experiments showed that REF seems to penetrate deeply into the monolayer and SRPP only binds to the lipid surface. These results could therefore clarify the role of these two paralogous proteins in latex production, either in the coagulation of natural rubber or in stress-related responses. To our knowledge, this is the first report of an amyloid formed from a plant protein. This suggests also the presence of functional amyloid in the plant kingdom.

The rubber tree genome reveals new insights into rubber production and species adaptation

“…Recent transgenic studies in Taraxacum brevicorniculatum 28,29 also reveal their essential role in rubber biosynthesis. REF and SRPP share a conserved REF motif that is also retained in several stress-related or lipid droplet-associated proteins in plant cells 25,30,31 . Based on the presence of a carboxy (C) terminus found in SRPP and its absence in REF 25,30 , and the similarities among protein sequences, the 18 Hevea REF/SRPP genes are named SRPP1 to 10 and REF1 to 8 ( Supplementary Fig.…”

Identification of a New Class of Lipid Droplet-Associated Proteins in Plants

“…Since the SRPP-like proteins appear to be mostly hydrophilic in nature, it will also be important to determine how these proteins associate with the lipid droplet surface, although recent studies of SRPP from Hevea spp. have yielded some important insight into this process (Berthelot et al, 2012). Furthermore, it will be important to identify proteins, if any, that interact with the SRPPs, as this may provide an avenue into a better understanding of the process of lipid droplet biogenesis in plant cells.…”

Lipid Droplet-Associated Proteins (LDAPs) Are Required for the Dynamic Regulation of Neutral Lipid Compartmentation in Plant Cells

“…This was somewhat unexpected, given that several discrete LD-targeting signals have been identified for LD proteins in a variety of organisms (DiNitto et al, 2003;Ingelmo-Torres et al, 2009;De Domenico et al, 2011). Furthermore, the REF protein of H. brasiliensis, which is similar in sequence to the N-terminal half of SRPP but lacks the corresponding C-terminal half, still effectively targets to and associates with rubber particles (Berthelot et al, 2012). Structural studies of REF and SRPP, however, suggest that the two rubber particle proteins adopt different conformations (Berthelot et al, 2012(Berthelot et al, , 2014a and thus may have evolved independent mechanisms for LD association.…”

“…Furthermore, the REF protein of H. brasiliensis, which is similar in sequence to the N-terminal half of SRPP but lacks the corresponding C-terminal half, still effectively targets to and associates with rubber particles (Berthelot et al, 2012). Structural studies of REF and SRPP, however, suggest that the two rubber particle proteins adopt different conformations (Berthelot et al, 2012(Berthelot et al, , 2014a and thus may have evolved independent mechanisms for LD association. Regardless, we showed that progressive deletions of the C-terminal region of LDAP3 effectively abolished LD association in vivo (Fig.…”