Bacterial Translocation and Protein Secretion

Translocation moves proteins from the cytoplasm to the plasma membrane, periplasm, or outer membrane, while secretion releases them into the extracellular environment.

The N-terminal signal sequences of most synthesized proteins direct their export via bacterial translocation systems.

The Sec and Tat systems are the common protein translocation systems in Gram-negative and Gram-positive bacteria.

The SecA protein identifies signal sequences on unfolded proteins destined for the periplasm or secretion, while the signal recognition particle — SRP, binds preproteins for membrane insertion.

SecA threads a preprotein through the SecYEG channel in the plasma membrane.

Contrastingly, the SRP-ribosome complex inserts its preprotein in the membrane via the SecYEG channel's lateral gate.

Finally, the Tat translocation system translocates fully folded proteins in the cytoplasm carrying cofactors before secretion.

TatBC, a docking complex, recognizes a distinctive twin-arginine signal sequence and escorts the folded protein to TatA — the membrane transporter, forming the TatABC complex for protein secretion.