Biology
- Animal Eukaryotic Cell Components: Structure And Function
I posted a picture of the eukaryotic animal cell earlier, but it was simply a labeled diagram. Here we will go into each of the major parts of the cell, as well as each organelle's function, not to mention the overall structure of the cell. As the...
- Reverse Translocation Sends Proteins To The Cytosol For Degradation
KEY TERMS:Retrograde translocation (Reverse translocation) is the translocation of a protein from the lumen of the ER to the cytoplasm. It usually occurs to allow misfolded or damaged proteins to be degraded by the proteasome. KEY CONCEPTS: Sec61...
- Signal Sequences Initiate Translocation
KEY TERMS:Protein sorting (targeting) is the direction of different types of proteins for transport into or between specific organelles. A signal sequence is a short region of a protein that directs it to the endoplasmic reticulum for co-translational...
- Passage Across A Membrane Requires A Special Apparatus
KEY TERMS:Protein translocation describes the movement of a protein across a membrane. This occurs across the membranes of organelles in eukaryotes, or across the plasma membrane in bacteria. Each membrane across which proteins are translocated has a...
- Eukaryotic Rnas Are Transported
KEY CONCEPTS:RNA is transported through a membrane as a ribonucleoprotein particle. All eukaryotic RNAs that function in the cytoplasm must be exported from the nucleus. tRNAs and the RNA component of a ribonuclease are imported into mitochondria. mRNAs...
Biology
PROTEIN LOCALIZATION
Proteins are synthesized in two types of location:
- The vast majority of proteins are synthesized by ribosomes in the cytosol.
- A small minority are synthesized by ribosomes within organelles (mitochondria or chloroplasts).
Proteins synthesized in the cytosol can be divided into two general classes with regard to localization: those that are not associated with membranes; and those that are associated with membranes (see 32.5 Membranes and membrane proteins). Figure 8.1 maps the cell in terms of the possible ultimate destinations for a newly synthesized protein and the systems that transport it:
- Cytosolic (or "soluble") proteins are not localized in any particular organelle. They are synthesized in the cytosol, and remain there, where they function as individual catalytic centers, acting on metabolites that are in solution in the cytosol.
- Macromolecular structures may be located at particular sites in the cytoplasm; for example, centrioles are associated with the regions that become the poles of the mitotic spindle.
- Nuclear proteins must be transported from their site of synthesis in the cytosol through the nuclear envelope into the nucleus.
- Most of the proteins in cytoplasmic organelles are synthesized in the cytosol and transported specifically to (and through) the organelle membrane, for example, to the mitochondrion or peroxisome or (in plant cells) to the chloroplast. (Those proteins that are synthesized within the organelle remain within it.)
- The cytoplasm contains a series of membranous bodies, including endoplasmic reticulum (ER), Golgi apparatus, endosomes, and lysosomes. This is sometimes referred to as the "reticuloendothelial system." Proteins that reside within these compartments are inserted into ER membranes, and then are directed to their particular locations by the transport system of the Golgi apparatus. (For an introduction see 32.6 ER and Golgi).
- Proteins that are secreted from the cell are transported to the plasma membrane and then must pass through it to the exterior. They start their synthesis in the same way as proteins associated with the reticuloendothelial system, but pass entirely through the system instead of halting at some particular point within it.
- Animal Eukaryotic Cell Components: Structure And Function
I posted a picture of the eukaryotic animal cell earlier, but it was simply a labeled diagram. Here we will go into each of the major parts of the cell, as well as each organelle's function, not to mention the overall structure of the cell. As the...
- Reverse Translocation Sends Proteins To The Cytosol For Degradation
KEY TERMS:Retrograde translocation (Reverse translocation) is the translocation of a protein from the lumen of the ER to the cytoplasm. It usually occurs to allow misfolded or damaged proteins to be degraded by the proteasome. KEY CONCEPTS: Sec61...
- Signal Sequences Initiate Translocation
KEY TERMS:Protein sorting (targeting) is the direction of different types of proteins for transport into or between specific organelles. A signal sequence is a short region of a protein that directs it to the endoplasmic reticulum for co-translational...
- Passage Across A Membrane Requires A Special Apparatus
KEY TERMS:Protein translocation describes the movement of a protein across a membrane. This occurs across the membranes of organelles in eukaryotes, or across the plasma membrane in bacteria. Each membrane across which proteins are translocated has a...
- Eukaryotic Rnas Are Transported
KEY CONCEPTS:RNA is transported through a membrane as a ribonucleoprotein particle. All eukaryotic RNAs that function in the cytoplasm must be exported from the nucleus. tRNAs and the RNA component of a ribonuclease are imported into mitochondria. mRNAs...