Protein Translocation Mechanisms across Cellular Membranes

Protein translocation across cellular membranes is fundamental to both prokaryotic and eukaryotic life, directing nascent polypeptides into organelles, the periplasmic space or the extracellular milieu. Central to this process is the translocon, a dynamic protein-conducting channel that collaborates with targeting factors such as the signal recognition particle to effect co-translational insertion or post-translational translocation. In eukaryotes, the endoplasmic reticulum membrane houses multiple pathways—including the Sec61 channel, the ER membrane protein complex and specialised insertases—that ensure precise insertion and correct topology of membrane proteins. Bacterial systems deploy Sec- and Tat-dependent routes, and emerging evidence supports a coupled transcription–translation–insertion mechanism known as transertion for rapid assembly of membrane complexes. Advances in structural and biochemical methods have begun to reveal how these machineries interconnect to govern proteome distribution, quality control and cellular homeostasis, with wide implications for understanding disease and exploiting biotechnological applications.

Research from Nature Portfolio

High-resolution cryo-electron tomography of native endoplasmic reticulum vesicles has captured snapshots of translating ribosomes engaged with variant translocon assemblies. The reconstructions reveal a predominant ER translocon comprising Sec61, the translocon-associated protein complex and the oligosaccharyltransferase complex, with stoichiometric cofactors that likely include protein isomerases. Distinct translocon specialisations accommodate either secretory precursors or multipass membrane proteins, and structural models suggest sustained association of eukaryotic elongation factor 1A with ribosomes beyond GTP hydrolysis, implying a proofreading or regulatory role during nascent chain maturation.

In bacterial systems, evidence for transertion has been provided by studies of Vibrio parahaemolyticus, which assembles its type III secretion system via a two-step programme of membrane-localised gene activation, translation and insertion. Bile acids activate a membrane-bound receptor complex, triggering expression of a transmembrane transcriptional activator that in turn coordinates localised synthesis and membrane insertion of secretion machinery components. This work supports a broader model in which coupled transcription–translation–insertion expedites the assembly of large membrane-associated complexes in response to environmental cues.

Protein Translocation Mechanisms across Cellular Membranes publication trend

The graph below shows the total number of articles in protein translocation mechanisms across cellular membranes across all publications each year (not limited to Nature Index journals).

Technical terms

Translocon: A membrane-embedded channel that conducts polypeptides across or into lipid bilayers.

Signal peptide: A short N-terminal sequence that directs nascent proteins to translocation pathways.

Signal recognition particle (SRP): A ribonucleoprotein that recognises hydrophobic targeting signals and delivers ribosome–nascent chain complexes to membranes.

Endoplasmic reticulum (ER): A membranous organelle in eukaryotes where secretory and membrane proteins are synthesised and processed.

ER membrane protein complex (EMC): A conserved multisubunit insertase that facilitates co-translational insertion and topogenesis of multipass membrane proteins.

Transertion: The coupled process of transcription, translation and membrane insertion of proteins at the site of their encoding genes.

References

1. Visualization of translation and protein biogenesis at the ER membrane. Nature (2023).
2. Membrane-localized expression, production and assembly of Vibrio parahaemolyticus T3SS2 provides evidence for transertion. Nature Communications (2023).
3. Mechanism of signal-anchor triage during early steps of membrane protein insertion. Molecular Cell (2023).
4. EMC Is Required to Initiate Accurate Membrane Protein Topogenesis. Cell (2018).