Describe the mechanisms which control the localisation of proteins to cellular compartments and the process reactions that occur within the compartments?
Features common to all proteins include the restraints placed on their conformation by covalent and noncovalent bonds (2001). The thousands of proteins present in the human body perform functions too numerous to list. One of these functions is associated with cellular compartments. There are many cellular compartments, each with its own specific features.
The protein biosynthetic pathways in cells can be considered to be one large sorting system. Many proteins carry signals (usually but not always specific sequences of amino acids) that target them to their destination, thus ensuring that they will end up in the correct membrane or cell compartment; these signals are a fundamental component of the sorting system (2000).
A major sorting decision is made early in protein biosynthesis, when specific proteins are biosynthesized either on free or on membrane-bound polyribosomes. This sorting occurs because proteins synthesized on membrane-bound polyribosomes contain a signal peptide that mediates their attachment to the membrane of the endoplasmic reticulum. Proteins synthesized on free polyribosomes lack this signal peptide and are delivered into the cytosol. There they are directed to the mitochondria, nuclei, and peroxisomes by specific signals – or remain in the cytosol if they lack signal (2000).
Proteins synthesized and sorted in the rough endoplasmic reticulum branch include many destined for various membranes like that of the endoplasmic reticulum itself, Golgi apparatus, lysosomes and plasma membranes; and for secretion. Lysosomal enzymes are also included. Thus, such proteins may reside in the membranes or lumina of the endoplasmic reticulum or follow the major transport route of intracellular proteins to the Golgi apparatus (2001).
There are four routes that proteins follow to be inserted into the membranes of the endoplasmic reticulum: (a) cotranslational insertion, (b) synthesis on free polyribosomes and subsequent attachment to the endoplasmic reticulum membrane, (c) retention at the luminal aspect of the endoplasmic reticulum by specific amino acid sequences, and (d) retrograde transport from the Golgi apparatus (2000).
The mitochondria contain many proteins. Thirteen proteins are encoded by the mitochondrial genome and synthesized in that organelle using its own protein-synthesizing system. However, the majority is encoded by nuclear genes, and must be imported (2000).
There are major pathways of proteins destined for the mitochondrial matrix. Overall, it is apparent that proteins employ a variety of mechanisms and routes to attain their final destinations in the mitochondria.
Most proteins that are synthesized on membrane-bound polyribosomes and are destined for the Golgi apparatus or plasma membrane reach these sites inside transport vesicles. There is evidence that proteins destined for the membranes of the Golgi apparatus have specific signal sequences. On the other hand, most proteins destined for the plasma membrane or for secretion do not appear to contain specific signals, reaching these destinations by default (2000).
The entire pathway of endoplasmic reticulum à Golgi apparatus à plasma membrane is often called the secretory or exocytotic pathway. Most of the proteins reaching the Golgi apparatus or the plasma membrane are carried in transport vesicles. Other proteins destined for secretion are carried in secretory vesicles. These are prominent in the pancreas and certain other glands. Their mobilization and discharge are regulated and often referred to as “regulated secretion,” whereas the secretory pathway involving transport vesicles is called “constitutive” (2000).
Credit:ivythesis.typepad.com
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