The structural and functional coupling of two molecular machines, the ribosome and the translocon

Ribosomes synthesizing secretory and membrane proteins are bound to translocons at the membrane of the endoplasmic reticulum (ER). Both the ribosome and translocon are complex macromolecular machines whose structural and functional interactions are poorly understood. A new study by Pool (Pool, M.R. 2009. J. Cell Biol. 185:889–902) has now shown that the structure of the translocon is dictated by the identity of the protein being synthesized by the ribosome, thereby demonstrating that the two macromolecular machines are structurally coupled for functional purposes. The study also identifies an unexpected component in the apparent molecular linkage that connects the two machines, a discovery that shows the current view of translocon structure is oversimplified.