Endoplasmic Reticulum, Golgi Apparatus, And Lysosomes - Nature

Membranes and their constituent proteins are assembled in the ER. This organelle contains the enzymes involved in lipid synthesis, and as lipids are manufactured in the ER, they are inserted into the organelle's own membranes. This happens in part because the lipids are too hydrophobic to dissolve into the cytoplasm.

Similarly, transmembrane proteins have enough hydrophobic surfaces that they are also inserted into the ER membrane while they are still being synthesized. Here, future membrane proteins make their way to the ER membrane with the help of a signal sequence in the newly translated protein. The signal sequence stops translation and directs the ribosomes — which are carrying the unfinished proteins — to dock with ER proteins before finishing their work. Translation then recommences after the signal sequence docks with the ER, and it takes place within the ER membrane. Thus, by the time the protein achieves its final form, it is already inserted into a membrane (Figure 1).

The proteins that will be secreted by a cell are also directed to the ER during translation, where they end up in the lumen, the internal cavity, where they are then packaged for vesicular release from the cell. The hormones insulin and erythropoietin (EPO) are both examples of vesicular proteins.

Figure 1: Co-translational synthesisA signal sequence on a growing protein will bind with a signal recognition particle (SRP). This slows protein synthesis. The SRP then binds to a location on the surface of the nearby ER. Then, the SRP is released, and the protein-ribosome complex is at the correct location for movement of the protein through a translocation channel.© 2014 Nature Education All rights reserved. Figure Detail