Secretion in Organisms

Secretion

  • Secretion occurs in animals, plants, and microorganisms.
  • Examples include fungal sex pheromones, plant gibberellins, and mammalian growth hormone.

Why Secrete Proteins?

  • For cell wall construction (microbes, plants).
  • For enzyme-mediated extracellular degradation of nutrients.
  • For cell communication (sex pheromones and hormones).

Mammalian Hormones

  • Hormones coordinate changes and are distributed by the circulatory system.
  • Endocrine cells form endocrine glands.

Mammalian Growth Hormone (GH)

  • GH promotes growth by stimulating amino acid uptake.
  • It stimulates the liver to produce insulin-like growth factors for bone and cartilage growth.
  • Overproduction causes gigantism; underproduction causes pituitary dwarfism.
  • GH is now produced using genetically engineered bacteria.

Plant Growth Hormone - Gibberellins

  • Gibberellins trigger enzyme secretion to digest proteins and starch.
  • They are secreted by the plant embryo and diffuse into fruit tissue.
  • Gibberellins are organic compounds, not proteins.
  • Gibberellin A1 controls stem elongation; treating dwarf plants restores normal growth.

Fungal Sex Pheromones

  • They allow fungi to recognize cells of the opposite mating type and promote mating.
  • Examples include yeast a-factor and yeast alpha-factor.

Yeast Alpha-Factor

  • It is a short peptide processed from a longer polypeptide.
  • Secreted by yeast alpha cells and detected by yeast a cells, promoting mating.

Protein Targeting and Secretion

  • Proteins are transported from the cytoplasm to cellular compartments after synthesis.
  • Compartments include the nucleus, mitochondrion, lysosome, chloroplast (intracellular), and periplasm, cell wall, bloodstream (extracellular).

The Secretory Pathway

  • Proteins move from the endoplasmic reticulum to the Golgi apparatus, vesicles, lysosomes, and plasma membrane.

Mechanisms of Secretion

  • Signal sequences indicate where the polypeptide belongs.
  • Secreted proteins contain an N-terminal Signal Sequence of circa 25 amino acids.
  • During translation, the signal sequence binds to the Signal Recognition Particle (SRP), stalling translation.
  • The complex docks at a receptor on the ER surface, opening a channel.
  • SRP disassociates, translation restarts, and the protein is translocated into the ER lumen.
  • The signal sequence is cleaved.
  • Chaperonins refold the protein in the ER.
  • Translation terminates, and the completed polypeptide is released into the ER lumen.

Post-Translational Events

  • Specific retention signals keep proteins in the ER.
  • Sugars are added in the Golgi to form glycoproteins.
  • Proteins with no further signals are secreted from the cell.
  • Common modifications include proteolysis, glycosylation, and phosphorylation.