03 Intracellular distribution of Proteins

03 Intracellular distribution of Proteins - Intracellular...

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Intracellular distribution of Proteins Many proteins are located and carry out their function in one specific cellular compartment. Proteins may be cytoplasmic or they may function in an organelle. They may be inserted into membranes , remaining inside the cell in the endoplasmic reticulum, or being inserted in the outer membrane. They may enter endoplasmic reticulum vesicles and be transferred to lysosomes , or be secreted from the cell. Other proteins may function in the mitochondria (and also chloroplasts in plants) and may be located in one of several different places in these organelles. Yet other proteins may function in the nucleus. All (or nearly all) are synthesised by ribosomes in the cytoplasm of the cell. How are they directed specifically to their appropriate cellular location?
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Cytoplasmic Proteins Nearly all proteins are synthesised by ribosomes in the cytoplasm, using a coded message transcribed from a gene in the nucleus as a strand of messenger RNA. The messenger RNA strand is usually long enough for several ribosomes to work on it at the same time. This group of ribosomes actively translating a mRNA message is called a polyribosome (or polysome). Proteins synthesised by polysomes may be directed to organelles, but most stay within the cytoplasm, functioning as cytoplasmic proteins. These proteins are usually not glycosylated (ie do not have sugar molecules attached).
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Rough Endoplasmic Reticulum Many ribosomes in cells are found associated with the endoplasmic reticulum. Because of its appearance in electron micrographs, endoplasmic reticulum with associated ribosomes is called rough endoplasmic reticulum and is a site of protein synthesis. Proteins synthesised here are destined to be transferred to lysosomes and similar organelles (but not mitochondria, chloroplasts or nuclei), into secretory vesicles, or located into the outer cell membrane. Proteins synthesised here all carry a signal sequence which directs them to be inserted into the endoplasmic reticulum.
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Signal Sequence Function A signal sequence is a length of amino acids from 10 – 40 residues long at the N-terminal end of a translated protein. Signal sequences on different proteins are not identical, but show similarities. All start with a methionine (coded by the AUG ‘start message’ codon) followed by 2 – 5 charged residues. This region is followed by a block of mostly uncharged hydrophobic residues 8 – 14 residues long followed by about 5 more hydrophillic residues. The translation of the messenger RNA coding for a protein with a signal sequence starts in the normal way with a ribosome attaching to the mRNA and decoding its message starting with the ‘start’ codon AUG. Translation proceeds until about 80 – 100 amino acids have been linked
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03 Intracellular distribution of Proteins - Intracellular...

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