16.1
Overview of Protein Sorting and Transport
Eukaryotic cells have different membrane-bound organelles with distinct protein requirements. The process by which proteins are targeted to a specific…
Protein sorting occurs during translation or after protein synthesis by two distinct processes: signal-based sorting and vesicle-based trafficking. Signal-based sorting employs two modes of transport: gated transport and protein translocation.
Nuclear proteins are actively transported from the cytosol to the nucleus via the nuclear pore complexes embedded in the nuclear envelope. This process is called gated transport.
These proteins contain specific amino acid sequences called sorting signals that are recognized by a sorting receptor that transports the protein to the nucleus.
Proteins targeted to the chloroplasts, mitochondria, and endoplasmic reticulum are imported through translocons, specialized transmembrane proteins, in a process called protein translocation.
During translocation, chaperone proteins in the cytosol bind to the target protein and deliver it to the membrane, where the sorting signals are recognized by the import receptors of the translocon. Using ATP hydrolysis, the target protein is unfolded and transported through a channel in the translocon to reach its destination within the organelle.
In vesicular trafficking, soluble proteins are packed from the endoplasmic reticulum lumen and loaded onto membrane-bound transport vesicles.
Transport vesicles bud off from the endoplasmic reticulum membrane and fuse with the Golgi membrane or the cell membrane without crossing the lipid bilayers to deliver the proteins to their target location.
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Q1: What are the two main processes of protein sorting in eukaryotic cells?
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.
Q2: How does gated transport differ from protein translocation?
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.
Q3: What role do chaperone proteins play in protein translocation?
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.
Q4: How do transport vesicles deliver proteins without crossing lipid bilayers?
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.
Q5: What is the function of sorting signals in protein targeting?
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.
Q6: Why is ATP hydrolysis necessary during protein translocation?
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.
Q7: Which organelles receive proteins through protein translocation?
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.