Nat. Plants 1, 15048 (2015);

J. Biol. Chem. 290, 1898–1914 (2015)

Credit: NATURE PLANTS

Natural rubber is a polymer of isoprene made by rubber cis-prenyltransferase (CPT) complexes. Failed attempts to reconstitute rubber biosynthesis via heterologous expression of CPTs have suggested that crucial elements of the complex remain unknown; the localization of CPTs at the surface of rubber particles suggests that this missing component includes a membrane-anchoring domain. In eukaryotes, CPTs also synthesize dolichol chains, shorter polymers necessary for glycoprotein synthesis; in this system, the integral membrane protein Nogo-B receptor protein (NgBR), that displays some homology to CPTs but is not an active enzyme, has recently been shown to interact with and stabilize CPTs. Epping et al. and Qu et al. suspected that an NgBR-like protein might also have a role in long-chain polymer synthesis. To test this hypothesis, Epping et al. first examined digested rubber particles from the dandelion Taraxacum brevicorniculatum, finding four peptides from an expressed sequence tag homologous to NgBR-like proteins that they named rubber transferase activator (RTA). TbRTA mRNA was most abundant in latex, and two tagged constructs further localized the protein in the rubber particle phase of separated latex and at least transiently on the ER membrane. TbRTA interacts with the CPT enzymes, as shown in planta by fluorescence analysis and co-immunoprecipitation experiments and in yeast by FRET analysis. Knockdown of TbRTA yielded plants that lacked CPT proteins, even though the corresponding mRNAs were present, and had empty rubber particle–like structures that did not produce rubber, though the cells continued to produce dolichol. Qu et al. performed similar experiments in the lettuce Lactuca sativa, finding that CPTL2, an NgBR-like sequence that is inactive but required for rubber biosynthesis, is localized on the ER and forms a stabilizing complex with the newly discovered enzyme CPT3. These combined results lead to a model in which NgBR-like proteins both stabilize CPTs and anchor them to the rubber particle to facilitate rubber synthesis.