Bacterial protein secretion involves translocation systems to ensure proteins reach their designated locations, including the plasma membrane, periplasm, outer membrane, or the external environment. These translocation systems are vital for bacterial physiology, supporting processes like membrane assembly, enzymatic activity in the periplasm, and interactions with the external environment. The division of labor between Sec and Tat pathways ensures efficiency in handling proteins with diverse folding states and functional requirements. These two systems are universal in bacteria and also seen in archaea.
Signal Sequences and Protein Targeting
Most proteins destined for export are synthesized with N-terminal signal sequences, which serve as molecular tags for export machinery. These sequences direct proteins to specialized translocation systems, facilitating their delivery to specific regions of the cell membranes or their secretion outside the cell.
The Sec Translocation System
The Sec pathway is the primary route for translocating unfolded proteins. The SecA motor protein recognizes signal sequences on preproteins targeted for the periplasm or secretion. Using ATP hydrolysis, SecA threads the preprotein through the SecYEG translocon, a channel embedded in the plasma membrane. Proteins destined for membrane insertion are handled differently; the signal recognition particle (SRP) binds these nascent preproteins as they emerge from the ribosome. The SRP-ribosome complex directs the preprotein to the SecYEG channel's lateral gate for integration into the lipid bilayer.
The Tat Translocation System
The twin-arginine translocation (Tat) system specializes in exporting fully folded proteins, often those containing cofactors essential for their function. Proteins designated for this pathway carry a distinctive twin-arginine motif in their signal sequence. TatBC, the docking complex, identifies and binds these signal sequences, directing the folded protein to TatA, the membrane transporter. Together, TatA, TatB, and TatC form the TatABC complex, enabling the transport of folded proteins across the membrane without compromising their structural integrity.
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.
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