EFB325 Cell Physiology

Protein sorting

Most polypeptides undergo further processing after translation

• in prokaryotes the chemical modification to the first methionine is removed, in eukaryotes the first methionine is usually cleaved off
• as translation proceeds, the polypeptide immediately begins the process of folding into its proper, stable 3-D structure, this is often aided by chaperone proteins, which act to prevent improper folding
• chemical modifications to particular amino acids can occur, including methylation, phosphorylation, acetylation, or within the ER and Golgi - glycosylation (the addition of carbohydrate groups)
• prosthetic groups (such as heme groups or metal atoms) are bound to some proteins and disulfide bridges can form (in the ER)
• many proteins are cleaved to generate a smaller polypeptide, which is the active protein

Although some of the organelles (mitochondria and chloroplasts) contain DNA and can synthesize some proteins internally, most of the proteins in the mitochondria and chloroplasts and all of the proteins in the ER, nucleus, lysosomes, peroxisomes, and vacuoles (in plants) are encoded by nuclear genes and are synthesized either in the cytosol or on the surface of the rough ER.

The proper organelle destination for a protein is determined by signals in the amino acid sequence itself

• there are particular sequences of amino acids that ultimately will bind to a receptor protein involved in importing that protein into the organelle
• in some cases, there is a single sequence at the N-terminus (beginning of the protein) in other cases, there are one or more internal sequences

1) Import through nuclear pores

• nuclear pores are large complex structures in the nuclear envelope that regulate movement of macromolecules (proteins & nucleic acids) in and out of the nucleus
• proteins bound for the nucleus have particular sequences of amino acids (nuclear localization signal) to which a nuclear import receptor protein will bind and direct the protein through the pore
• proteins bound for the nucleus are synthesized on free cytoplasmic ribosomes

2) Post-translational import into the mitochondria and plastids

Proteins bound for the mitochondria, chloroplast, and peroxisome are made on free cytoplasmic ribosomes and then make their way to the proper organelle = posttranslational import

• these proteins also have a special sequence of amino acids at the N-terminus of the protein called the signal sequence, which is recognized by receptor proteins in the membranes of the appropriate organelle
• the protein is unfolded, then the receptor protein and a translocator assist in the transport of the polypeptide across the membrane and into the organelle
• once the signal sequence is inside, it is cleaved off by a peptidase enzyme and the protein is folded into its final 3D shape, often with the assistance of molecular chaperones

3) Co-translational import into the ER

Proteins destined for secretion out of the cell, for the ER, Golgi, vacuole, lysosome, or plasma membrane are directed across the ER membrane as they are being synthesized = cotranslational import

• synthesis of these proteins starts (by ribosomes), but as soon as the N-terminus of the protein containing the signal sequence is made, a protein complex binds to it (signal recognition particle) and directs the protein and ribosome to a receptor protein in the ER membrane
• the signal sequence at the N-terminus and the rest of the growing polypeptide chain are directed through the ER membrane and into the ER
• an enzyme (a peptidase) in the ER cleaves the polypeptide to release the signal peptide

4) Vesicles bud off of the ER and Golgi, then fuse with specific organelles or the plasma membrane

• vesicles bound for different destinations are coated with different types of proteins that target them to the right location

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