# Biosynthesis of Natural Rubber: Current State and Perspectives
## 2 Biosynthesis of Natural Rubber and the Involved Genes/Proteins
### Structure, Components and Biogenesis of Rubber Particles  
The exact tissues where natural rubber is synthesized are different, depending on plant species. For example, rubber synthesis happens in specialized laticifer in phloem of _H. brasiliensis_, _T. kok-saghyz_ and _E. ulmoides_, whereas it takes place in gutta-containing cells derived from epithelial cells in parenchyma tissue and pith in _P. argentatum_[2; 29; 30; 24]. Through ultracentrifugation, latex can be separated in at least 3 compartments: the cream with rubber particles and Frey-Wissling particles, the cytoplasmic C-serum and the bottom fraction with lutoids [31]. Natural rubber biosynthesis happens on the surface of rubber particles which is one of the main components of latex (30-50% by volume in _H. brasiliensis_) [32]. Rubber particles are spherical or pear-shaped subcellular organelles that have diameter ranges of 0.02-3.0 \(\upmu\)m in _H. brasiliensis_, 1.0-2.0 \(\upmu\)m in _P. argentatum_, 0.2-1.0 \(\upmu\)m in _T. kok-saghyz_, 1.6-6.5 \(\upmu\)m in _Ficus_ species, and 0.02-1.2 \(\upmu\)m in _Euphoria charcias_[1; 33; 34; 35; 36; 37]. They are made of polyisoprene, surrounded by a monolayer of lipids with proteins and other compounds [1]. Proteins  
Figure 2: Proposed initiation and polymerization mechanism in rubber biosynthesis.  
involved in rubber synthesis associate with or were inserted into the lipidic monolayer and were thought to form a complex or work synergistically during the rubber biosynthesis [31; 38]. The synthesized polyisoprene chains were elongated inward and stored in the interior of the rubber particles, resulting in the enlargement of the rubber particles. Besides, proteins involved in stress resistance and rubber flocculation, allergic proteins and function unknown proteins were also found on rubber particles [37; 39; 40; 41].  
Rubber particles were supposed to originate from endoplasmic reticulum (ER) or Golgi according to studies of their structure and the proteins isolated from the rubber particles, which have typical ER or Golgi localization such as rubber elongation factor (REF), small rubber particle protein (SRPP), _cis_-prenyltransferase-like protein (CPTL), clathrin and Rab GTPases [1; 39; 42; 43; 44; 45]. This view was also supported by the findings that phosphatidylcholine, phosphatidylcholine and glycoproteins are abundant in the monolayer biomembrane, which is typical for the cytoplasmic part of the ER bilayer membrane [46]. It was postulated that after the first polyisoprene chain was synthesized by the rubber transferase complex localized on the ER, it was inserted into the hydrophobic region between the lipid bilayer. When more polyisoprene chains were synthesized, the region formed a convex outward with budding from the ER into the cytosol, and was finally separated from the ER membrane and generated the rubber particles [1; 29; 47; 48].  
Rubber particles can be classified into small rubber particles (SRPs) and large rubber particles (LRPs) [35]. Besides the size difference, SRPs were demonstrated to have much higher rubber biosynthesis activity than LRPs by in vitro assays, which may be due to the different protein profiles between them [37; 49; 50]. Moreover, the stored natural rubber molecules inside the rubber particles were supposed to have different architectures between SRPs and LRPs. SRPs are mainly composed of linear molecules with no or few branches, while LRPs contain branched molecules with join points originating from phospholipids [51].