Translocon

A translocon is a protein complex embedded in a biological membrane that forms a pore through which polypeptides can cross the membrane or integrate into it. Translocons are essential for the cellular secretory pathway, enabling newly synthesized proteins to enter organelles such as the endoplasmic reticulum (ER) or to become embedded in membranes.

In bacteria, the core translocon is the SecYEG complex, which forms a channel through the inner membrane. In archaea and eukaryotes, the homologous complex is SecY, SecE, and SecG in different arrangements, commonly referred to as SecYEG or Sec61 in eukaryotes. Protein translocation through these channels is typically targeted by signal sequences on nascent polypeptides and is assisted by cytosolic or membrane-associated factors. In bacteria, the SecA ATPase provides the driving force for post-translational translocation, while in eukaryotes and most organelles, ribosome docking with the Sec61 channel or associated chaperones such as BiP helps pull the polypeptide through the channel during co-translational translocation.

Beyond the ER, specialized translocons handle protein import into other organelles. The mitochondrial and chloroplast protein import systems (such as TOM/TIM in mitochondria and TIC/TOC in chloroplasts) are multi-subunit channels that recognize precursor proteins and facilitate their passage across and into organelle membranes. In the ER, additional components like the TRAP complex and other auxiliary factors participate in membrane protein insertion and processing.

Overall, translocons are conserved, multi-subunit assemblies that coordinate recognition, unfolding, directionality, and insertion or translocation of proteins across or into membranes, enabling proper protein localization and function.