Translokons

A translokon is a protein-conducting channel that spans the membrane of the endoplasmic reticulum. It plays a crucial role in the translocation of newly synthesized polypeptides from the cytosol into the lumen of the endoplasmic reticulum or into the ER membrane itself. The translokon is a dynamic structure, typically composed of multiple protein subunits, and its precise composition can vary depending on the type of protein being translocated.

The process of translocation mediated by the translokon begins when a ribosome synthesizing a protein with a signal sequence emerges from the cytosol. This signal sequence is recognized by the signal recognition particle (SRP), which then escorts the ribosome-nascent polypeptide complex to the ER membrane. Here, the SRP binds to the SRP receptor, bringing the ribosome into close proximity with the translokon. The signal sequence inserts into the translokon channel, triggering its opening.

As the ribosome continues protein synthesis, the polypeptide chain is threaded through the translokon pore in an unfolded or partially folded state. Once the polypeptide emerges into the ER lumen, the signal sequence is often cleaved by a signal peptidase. The translokon then closes, allowing the ribosome to dissociate and resume protein synthesis in the cytosol. This mechanism ensures that proteins destined for secretion, insertion into membranes, or delivery to other organelles within the secretory pathway are correctly directed and protected from degradation in the cytosol. The proper functioning of translokons is essential for cellular homeostasis and the production of a wide array of functional proteins.