Question

- Design a set of experiments to prove that a certain sequence is sufficient to to deliver a certain protein to the ER.

-Using site-directed mutagenesis, how could you identify the residues within the signal sequence that are:

1) most important for delivery to the ER and 2) necessary for signal peptidase activity?

Answer 1

Different proteins need to be sent to the different parts of a eukaryotic cell. In some cases, they are exported out of the cell and then into extracellular space. Cells have various kind of shipping systems, like molecular versions of postal service, to make sure that proteins arrive at the correct destination.

Different shipping routes

Translation of all proteins in a eukaryotic cell begins from cytosol. As a protein is made, it passes away the step by step through a shipping "decision tree." At each stage, the protein is checked for a molecular tags to see if it needs to be re-routed to a different pathway.Proteins all begin their synthesis in the cytosol. Many stay there permanently, but some are transported to other cellular destinations.

The major part of shipping routes starts after translation starts. At this point, the protein will either remain in the cytosol for the rest of translation. Proteins are need to be fed into the ER during translation if they have an amino sequence called a signal peptide. Proteins are bound for organelles in the endomembrane system

• Proteins that do not have a signal peptide stay in the cytosol for the rest of translation. If they lack other "address labels," they'll stay in the cytosol permanently. However, if they have the right labels, they can be sent to the mitochondria, chloroplasts, peroxisomes, or nucleus after translation.
• Proteins destined for any part of the endomembrane are brought to the ER during translation and fed in as they are made.

MOST IMPORTANT DELIVERY TO THE ENDOPLASMIC RECTICULUM

Signal peptide

The signal peptide that sends a protein into the endoplasmic reticulum during translation is a series of hydrophobic amino acids, usually found near the beginning of the protein. When this sequence sticks out of the ribosome, it’s recognized by protein complex called the signal-recognition particle (SRP), which takes the ribosome to the ER. There, the ribosome feeds its amino acid chain into the ER lumen (interior) as it's made. Signal recognition particle (SRP) binds to the signal peptide as it emerges from the ribosome. SRP brings the ribosome to the ER by binding to a receptor on the ER surface. The receptor is associated with other proteins that make a pore. The ribosome resumes translating and feeding the polypeptide through the pore and into the ER lumen.

Transport through the endomembrane system

In the ER, proteins fold into their correct shapes, may also get sugar groups attached to them. Most proteins are then transported to Golgi apparatus in membrane vesicles. Some proteins, need to stay in the ER.

Targeting to non-endomembrane organelles

Proteins that are made in the cytosol may stay permanently in the cytosol. Proteins bound for the mitochondria, chloroplasts, peroxisomes, and nucleus are usually made in the cytosol and are then delivered after the translation is complete.

Mitochondrial, chloroplast, and nuclear targeting are generally similar to peroxisomal targeting. That is, a certain amino acid sequence sends the protein to its target organelle .

Different proteins need to be sent to the different parts of a eukaryotic cell. In some cases, they are exported out of the cell and then into extracellular space. Cells have various kind of shipping systems, like molecular versions of postal service, to make sure that proteins arrive at the correct destination.

Different shipping routes

Translation of all proteins in a eukaryotic cell begins from cytosol. As a protein is made, it passes away the step by step through a shipping "decision tree." At each stage, the protein is checked for a molecular tags to see if it needs to be re-routed to a different pathway.Proteins all begin their synthesis in the cytosol. Many stay there permanently, but some are transported to other cellular destinations.

The major part of shipping routes starts after translation starts. At this point, the protein will either remain in the cytosol for the rest of translation. Proteins are need to be fed into the ER during translation if they have an amino sequence called a signal peptide. Proteins are bound for organelles in the endomembrane system

• Proteins that do not have a signal peptide stay in the cytosol for the rest of translation. If they lack other "address labels," they'll stay in the cytosol permanently. However, if they have the right labels, they can be sent to the mitochondria, chloroplasts, peroxisomes, or nucleus after translation.
• Proteins destined for any part of the endomembrane are brought to the ER during translation and fed in as they are made.

MOST IMPORTANT DELIVERY TO THE ENDOPLASMIC RECTICULUM

Signal peptide

The signal peptide that sends a protein into the endoplasmic reticulum during translation is a series of hydrophobic amino acids, usually found near the beginning of the protein. When this sequence sticks out of the ribosome, it’s recognized by protein complex called the signal-recognition particle (SRP), which takes the ribosome to the ER. There, the ribosome feeds its amino acid chain into the ER lumen (interior) as it's made. Signal recognition particle (SRP) binds to the signal peptide as it emerges from the ribosome. SRP brings the ribosome to the ER by binding to a receptor on the ER surface. The receptor is associated with other proteins that make a pore. The ribosome resumes translating and feeding the polypeptide through the pore and into the ER lumen.

Transport through the endomembrane system

In the ER, proteins fold into their correct shapes, may also get sugar groups attached to them. Most proteins are then transported to Golgi apparatus in membrane vesicles. Some proteins, need to stay in the ER.

Targeting to non-endomembrane organelles

Proteins that are made in the cytosol may stay permanently in the cytosol. Proteins bound for the mitochondria, chloroplasts, peroxisomes, and nucleus are usually made in the cytosol and are then delivered after the translation is complete.

Mitochondrial, chloroplast, and nuclear targeting are generally similar to peroxisomal targeting. That is, a certain amino acid sequence sends the protein to its target organelle .