Protein Secretion and Hormonal Regulation

Protein Secretion and Hormonal Control

Constructing extracellular structures:
- Cell walls in microbes and plants require release of materials for external buildup.
- Fungi utilize this process.
Degrading extracellular substances:
- Digestive enzymes and other enzymes act outside the cell.
Communication:
- Hormones facilitate cell-to-cell communication.
  - Pheromones between microbes.
  - Hormones within multicellular organisms (released into the bloodstream).

Numerous hormones act on the whole body or specific tissues.
- Local action: Dopamine acts within the brain.
- Global action: Hormones released into the bloodstream affect the entire organism.
Hormones must be secreted to act on distant cells.
They diffuse into extracellular fluid for local action or enter the bloodstream for systemic effects.
Target organs possess receptors on their cell surfaces that recognize specific signal molecules.
Hormones regulate:
- Metabolic processes.
- Growth of cells.
- Physiological processes like heart rate and blood pressure.
Specialized organs (endocrine glands) often produce hormones.

Growth hormone, produced in the pituitary gland, regulates growth from childhood to adulthood.
It enables cells to take up amino acids, crucial for cell division during growth.
It stimulates the liver to produce growth factors, such as insulin-like growth factor (IGF).
- IGF levels correlate with height.
Examples:
- Gigantism (excess growth hormone).
- Pituitary dwarfism (low growth hormone).
Low growth hormone levels can be treated with bacterially produced growth hormone.
- The human growth hormone gene is cloned and expressed in bacteria.
- The hormone is then injected to restore functionality.

Gibberellins trigger seed germination and growth.
They break down starch (a glucose polymer) within the seed to provide energy for cell division.
- Starch \rightarrow Glucose
Gibberellin A1 controls plant height.

Sex pheromones enable individual cells to recognize each other for mating.
This process maintains genetic diversity within a population.
Examples:
- A factor and alpha factor in yeast.
  - These allow cells to recognize opposing mating types.
Alpha factor is a small peptide that triggers cell recognition and fusion.
A signal sequence allows the protein to be secreted from the cell.

Protein secretion involves targeting proteins to specific pathways to exit the cell.
This requires proper protein folding and transport across the cell membrane.

Proteins begin to fold as soon as they are generated.
Translation happens in the cytoplasm or rough endoplasmic reticulum (RER).
Proteins synthesized in the cytoplasm float off and function within the cell.
Proteins synthesized in the RER are processed through the RER and Golgi apparatus.

The RER and Golgi are layered organelles that sort proteins.
Proteins move between layers via budding and are folded continuously.
They are sorted and stored in lysosomes.
Targeting signals on proteins direct them to specific destinations within the cell.

mRNA is translated by ribosomes, generating proteins.
Signal sequences, short amino acid sequences, target proteins for secretion.
The signal recognition particle (SRP) recognizes these sequences.
The SRP targets the ribosome to the surface of the endoplasmic reticulum.
The ribosome comes into contact with a receptor protein on the ER surface.
As the protein is generated, it enters the endoplasmic reticulum through a channel.

Translocation is the movement of the newly synthesized protein into the endoplasmic reticulum.
After the signal sequence passes into the ER, it is cleaved off by enzymes.
Chaperone proteins help the protein fold correctly within the ER.
The translation process continues until the entire RNA sequence is read.
Once synthesized and folded, additional signals direct the protein to specific organelles or processes.
Retention signals keep proteins in the ER if their function requires it.

Glycoproteins, proteins with added carbohydrates, are formed within the Golgi apparatus.
The Golgi apparatus facilitates the synthesis, sorting, and modification of proteins for release onto the cell surface.

Proteins undergo modifications after translation to achieve their final functional form.

Proteolysis:
- Cleaving a portion of the protein structure.
- Example: Insulin synthesis (preproinsulin → proinsulin → active insulin).
Glycosylation:
- Adding sugars (small or large carbohydrate complexes).
Phosphorylation:
- Adding phosphate groups to confer activity.
- Often proteins undergo all these modifications.