Protein targeting

Protein targeting is the recognition and transport of proteins to particular cellular destinations, including organelles, membranes, and sites of secretion.

Signal peptide: A short amino-acid sequence that directs a protein to a particular cellular destination. Many proteins entering the secretory pathway expose this sequence for recognition.
Protein sorting signal: A sequence or structural feature in a protein that determines its cellular destination. Targeting depends on signals that distinguish proteins bound for different destinations.
Protein secretion: The release of proteins from cells through vesicular export or direct membrane-translocation pathways. Targeting routes secreted proteins into the pathway that carries them outside the cell.
Protein mislocalization: The presence of a protein in a cellular location where it does not normally function. Failed or altered targeting can place proteins in the wrong compartment and impair their activity.
Signal recognition particle: A ribonucleoprotein complex that recognizes signal peptides and helps target translating ribosomes to the endoplasmic reticulum. It links signal-peptide recognition to cotranslational delivery at the ER.
Endoplasmic reticulum: A membrane network where many proteins are synthesized, folded, and modified. It is the entry point for proteins bound for secretion and much of the endomembrane system.
Mitochondrial protein import: The transport of proteins, mostly synthesized in the cytosol, into mitochondria or their membranes. It is a major targeting pathway for proteins encoded by nuclear genes.
Endoplasmic-reticulum-associated degradation: A quality-control pathway that recognizes misfolded ER proteins, moves them to the cytosol, and directs them for degradation. Proteins that fail folding after ER targeting can be removed through this pathway.
Sec61 translocon: A protein-conducting channel in the endoplasmic-reticulum membrane that allows proteins to enter or cross the membrane. Secretory proteins pass through this channel as they are synthesized.
Nuclear pore complex: A large protein assembly embedded in the nuclear envelope that regulates transport between nucleus and cytoplasm. Nuclear targeting ends at this selective gateway rather than at a membrane translocon.

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