Proteins are largely responsible for an organism’s structure and function. In organisms with an organized nucleus, known as eukaryotes, the ribosomes and the rough endoplasmic reticulum, or ER, play important roles in the synthesis of proteins. The point of attachment between a ribosome and the ER is a sophisticated pore known as a translocon. It’s the job of the translocon to grab ribosomes and allow newly minted proteins to enter the ER.
The Endoplasmic Reticulum
The ER is a set of tubes and sacs, called cisternae, enclosed in a network of membranes. The ER extends from the outer surface of the nuclear membrane into the cell body. Rough ER is a host for ribosomes that continually attach to and detach from the ER surface. The rough ER’s main function is to help form and store proteins, while smooth ERs store lipids, a type of fat. Many of the proteins created by attached ribosomes pass into the rough ER and then travel to other parts of the body.
Ribosomes are composed of ribosomal RNA and proteins. They are manufactured in the cell nucleus in two types of subunits, the large and the small. The subunits transfer to the cell body, where they float free in the cytoplasm or attach to the rough ER. Ribosomes read strands of messenger RNA and bind matching units of transfer RNA to the currently read portion. The ribosome and its associated enzymes transfer an amino acid from the transfer RNA to an elongating length of protein in a process called translation.
The translocons are tiny docking stations on the rough ER surface that lock onto ribosomes. When a ribosome begins making proteins, the translocon opens enough for the newly created protein to feed into the pore. The new protein passes into the pore in a linear or helical form, because the pore is too small to allow a folded protein to pass within. The translocon pore only opens if it recognizes a special sequence of amino acids that ribosomes use to start a newly created protein.
Fate of the Protein
The translocon controls whether the new protein will be incorporated into the plasma membrane or will be stored in soluble form within the ER. The proteins that enter the tight confines of the ER membranes get bent and folded into their characteristic final shapes. These shapes result in part from atomic bonds between different portions of the protein molecule. The ER performs quality control by transporting abnormal or misshaped proteins back into the cell body where they are recycled. Stored proteins travel into another cell organelle, called the Golgi apparatus, and eventually exit the cell. When the ribosome finishes synthesizing a protein, the translocon ejects the ribosome and plugs up the pore.