Overview of Protein Sorting and Transport

Protein sorting occurs through signal-based sorting and vesicle-based trafficking. Signal-based sorting uses specific amino acid sequences called sorting signals to direct proteins via gated transport or protein translocation. Vesicle-based trafficking moves soluble proteins in membrane-bound transport vesicles that bud from the endoplasmic reticulum and fuse with target organelles without crossing lipid bilayers.

Gated transport moves folded nuclear proteins through nuclear pore complexes using nuclear localization signals recognized by sorting receptors. Protein translocation, by contrast, requires unfolding the target protein and threading it through translocons to cross organelle membranes. Translocation targets proteins to mitochondria, chloroplasts, and the endoplasmic reticulum using energy from ATP hydrolysis.

Cytosolic chaperone proteins bind to target proteins in the cytosol and unfold them, delivering them to the organelle membrane. These chaperones guide the unfolded protein precursor to import receptors on the translocon, which recognize the signal sequences. This process enables the protein to be threaded through the translocon channel into the organelle lumen.

Transport vesicles bud from the endoplasmic reticulum membrane with soluble proteins packed in their lumen. These membrane-bound structures fuse directly with target organelle membranes or the cell membrane, transferring their cargo. Molecular markers displayed on organelle membranes guide the proper delivery and fusion of transport vesicles to their destinations.

Sorting signals are specific amino acid sequences on proteins that direct them to proper cellular locations. These signals are recognized by sorting receptors that bind the cargo and transport it to the correct organelle. In gated transport, sorting signals are exposed on folded proteins; in translocation, they are recognized on unfolded protein precursors by import receptors on translocons.

ATP hydrolysis provides energy to unfold target proteins and drive them through the translocon channel across the organelle membrane. This energy-dependent process allows the unfolded peptide chain to be threaded through the specialized transporter against resistance. Without ATP hydrolysis, proteins cannot be successfully transported through translocons into mitochondria, chloroplasts, or the endoplasmic reticulum.

Proteins are translocated into mitochondria, chloroplasts, the endoplasmic reticulum, and peroxisomes. Each organelle has specific import receptors on its membrane that recognize signal sequences on unfolded protein precursors. The translocation of proteins into the mitochondria involves threading unfolded proteins through translocons in the organelle membrane to reach their final destination.