Authors: Thomas Schmidt (equal contributor) ; Malte Lenders (equal contributor) ; Andrea Hillebrand ; Nicole van Deenen ; Oliver Munt ; Rudolf Reichelt ; Wolfgang Eisenreich ; Rainer Fischer ; Dirk Prüfer [1,4]; Christian Schulze Gronover (corresponding author) 
Natural rubber poly(cis -1,4-isoprene) with a molecular mass of 10-10,000 kDa is one of the most important industrial raw materials in the world, and its sole commercial source is currently the para rubber tree Hevea brasiliensis . Other sources, such as Russian dandelion (Taraxacum koksaghyz Rodin) and Guayule (Parthenium argentatum ), could be useful in the event of supply shortages as well as providing a suitable alternative for people with allergies to hevein, a major allergen present in H. brasiliensis latex [2, 3].
The biosynthesis of natural rubber takes place in the latex of laticifers or specialized parenchyma cells in the bark , where it is stored in rubber particles as an end product. Ultrastructural analysis of rubber particles from different species [4, 5, 6] revealed an almost identical globular structure that contains a homogeneous hydrophobic rubber core surrounded by an intact monolayer membrane. The monolayer membrane includes a mixture of lipids, proteins and other molecules with the hydrophilic portions of the phospholipids and glycosylated particle-bound proteins facing the cytoplasm [6, 7, 8, 9, 10]. The size of rubber particles ranges from 0.08-2 [mu]m in H. brasiliensis , 0.2-6.5 [mu]m in Ficus species and 1-2 [mu]m in P. argentatum [6, 11].
Natural rubber is synthesized by adding activated 2-methyl-1,3-butadiene (isopentenyl diphosphate, IPP) to the growing chain [12, 13]. This reaction is catalyzed by specific long-chain cis -prenyltransferases (CPTs, EC 184.108.40.206), which are probably located on the surface of rubber particles. According to their function, CPTs are classified as short-, medium- or long-chain polymerizing enzymes and can be distinguished from trans -prenyltransferases (TPTs) by the presence of five conserved protein motifs . They are found in bacteria , yeast , animals including humans  and plants [18, 19, 20, 21, 22].
Recently, two CPTs (RER2 and SRT1) were isolated from Saccharomyces cerevisiae and were shown to be responsible for the biosynthesis of dolichol, a long-chain polyprenol with a saturated alpha-isoprene unit, which serves as a glycosyl carrier for protein glycosylation in the endoplasmic reticulum . The first plant CPT was identified in Arabidopsis thaliana (ACPT), and appears to be required for normal growth and development . The latex of H. brasiliensis contains at least two CPTs, designated HRT1 and HRT2 (for Hevea rubber transferase). The addition of recombinant HRT2 to washed latex particles supplemented with radioactively-labeled IPP resulted in the significant production of a high-molecular-weight labeled rubber product, whereas recombinant HRT1 showed no significant activity . In vitro , initiation of rubber biosynthesis by HRT requires intact particles, isopentenyl diphosphate (IPP), allylic diphosphates such as farnesyl diphosphate (FPP) and divalent metal cations (Mg2+ or Mn2+ ) as a co-factor [26, 27, 28]. However, all attempts to purify a functional rubber transferase from rubber particles have failed, suggesting that the native enzyme needs additional factors for...
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