lecture 7 - Protein sorting and nuclear hormone receptor signalling

Question 1

What is the endomembrane system?

Answer 1

•A complex system of internal membranes and
membrane bound vesicles.
•Allows the formation of multiple sub-cellular
compartments with complex micro-environments for
organelles such as mitochondria and nucleus

The vesicle transport system has multiple
functions.
1. Protein secretion and transport
2. Protein processing and degradation
3. Nutrient acquisition
Endocytosis- Uptake of extracellular material
into membrane bound vesicles
Exocytosis- Release of material from membrane
bound vesicles into the extracellular space

Question 2

What is protein targeting/sorting?

Answer 2

•Specific proteins are required for the function of these
compartments
•Translation occurs in the cytoplasm
•Proteins frequently contain peptide motifs that are recognised by cellular machinery and target the proteins to different compartments of the cell.

Question 3

What is an NLS?

Answer 3

• A sequence of amino acids called a Nuclear Localisation Signal
(NLS) targets proteins translated in the cytoplasm to the
nucleus.
• Transcription factors typically contain an NLS

Question 4

Describe how protein secretion takes place

Answer 4

• Proteins that are either secreted from the cell or are inserted into cellular plasma membrane (e.g hormones and hormone receptors) are targeted to the endoplasmic reticulum (ER).
• Secreted proteins typically contain an amino terminal secretory signal sequence.
• Sequence contains 20 to 30 amino, with a hydrophobic core that allows the protein to insert into the ER membrane
• Signal peptide recognised by signal recognition particle (SRP)
• SRP binds to SRP receptor on the external surface of an ER vesicle
• Protein translocates into the internal lumen via the protein translocator
• Signal peptide is cleaved by a signal peptidase
• Protein is trafficked through the ER.

Question 5

Describe the role of the Golgi apparatus

Answer 5

• Membrane associated and secretory proteins which have passed into the ER are trafficked in vesicles to the Golgi apparatus.
• As proteins pass through the ER and Golgi they are frequently modified before being presented at the cell surface.

Question 6

What is glycosylation?

Answer 6

Occurs in the ER and Golgi
• The addition of sugar residues to protein chains
• Glycosylation of secreted proteins is very
common
• Glycosylation can stabilise proteins and allow
them to fold correctly

Question 7

What other post translational modifications take place?

Answer 7

•Protein folding, formation of intra and inter
molecular disulphide bonds.
•Attachment of lipid modifications to membrane
bound proteins.

Question 8

What are hydrophobic signalling molecules?

Answer 8

• Hydrophobic signalling molecules are insoluble in water and are transported bound to carrier proteins.
• At the cell the signal is released and diffuses across the plasma membrane to bind with a receptor protein in the cytoplasm or nucleus.
• Intracellular signal transduction pathway is typically less complex

Question 9

What is nuclear hormone receptor signalling?

Answer 9

Nuclear hormone receptors are used in signalling by a
large group of small hydrophobic molecules with diverse chemical structures
•Steroid hormones, thyroid hormones, retinoic acid and
vitamin D signal through nuclear hormones receptors
•Hydrophilic signals such as proteins are rapidly turned
over and degraded in the bloodstream and cellular
environment and are used for rapid signalling processes.
•Hydrophobic signals persist in the body much longer
and are involved in long term signalling.

Question 10

What are steroids?

Answer 10

•Include the sex hormones and cortisol.
•Sex hormones produced by the gonads
•Cortisol is produced by the adrenal cortex
•Steroid hormones are synthesized from cholesterol
•Typically act as endocrine hormones and are
transported systemically in the blood stream.

Question 11

How do nuclear hormone receptors become active?

Answer 11

•Nuclear hormone receptors are transcription factors
•In the absence of the signalling molecule (ligand) the
receptor is held in an inactive protein complex.
•Ligand binding leads to conformational change in the
receptor allowing it to become active and act as a
transcription factor.

The cellular localisation of the inactive receptor varies

Question 12

How is cortisol activated?

Answer 12

• In the absence of ligand the receptor is held in an inactive complex in the cytoplasm
• In the presence of the ligand the receptor is released from the complex and translocates to the nucleus.

Question 13

How is retinoic acid activated?

Answer 13

• Retinoic acid is a derivative of vitamin A involved in multiple processes in animal development e.g limb development
• In the absence of ligand the receptor is bound to regulatory regions of target genes in the nucleus in association with negative regulators of transcription
• In the presence of the ligand the receptor dissociates from the inhibitors and associates with activators