Week 11: Transmembrane Proteins and Cell Organelles

Week 11: Detailed Study Notes

Overview of Topics Covered

  • Focus areas: Transmembrane proteins, Endoplasmic Reticulum (ER), Golgi apparatus, coat proteins, endocytosis, autophagy, cytoskeleton, motor proteins, nucleus.

Transmembrane Proteins and Cellular Structures

  • Transmembrane Proteins: Integral proteins that span the membrane and facilitate the transport of substances in and out of the cell.

  • Endoplasmic Reticulum (ER):
      - Site of protein synthesis, modification, and transport.
      - Rough ER has ribosomes attached for protein synthesis.

Golgi Apparatus

  • Functions:
      - Receives proteins from the ER.
      - Modifies, sorts, and packages proteins for secretion or delivery to other organelles.
      - Comprised of a series of flattened membranous sacs (cisternae).

Coat Proteins

  • Role of coat proteins in vesicle formation and transport:
      - Coat Protein Complex II (COPII): Moves material from the ER to the Golgi (anterograde transport).
      - Coat Protein Complex I (COPI): Transports material from the Golgi back to the ER (retrograde transport).
      - Clathrin: Mediates transport from Golgi to endosomes and plasma membrane (associated with endocytosis).

Endocytosis

  • Definition: Process by which cells internalize substances from their external environment.

  • Steps in endocytosis: Membrane invagination forms a vesicle that contains the ingested substances. This vesicle can then fuse with lysosomes.

Cellular Organelles and Functions

Peroxisomes

  • Function: Break down fatty acids, detoxify harmful substances, and mitigate reactive oxygen species (ROS).

  • Importance of catalase enzyme: Breaks down hydrogen peroxide into water and oxygen to prevent oxidative damage.

Zellweger Syndrome

  • An autosomal recessive condition caused by dysfunctional peroxisomes leading to severe developmental defects, weak muscle tone, seizures, and typically a short lifespan (often less than one year).

  • Symptoms: Hypomelanation (deficient myelin), apneas (sleeping disorders), and renal problems.

Reactive Oxygen Species (ROS)

  • Hydrogen Peroxide (H₂O₂) can be beneficial in low concentrations (antimicrobial activity) but harmful in excess.

  • Controlled by catalase, reducing oxidative stress on the cell.

Cystic Fibrosis (CF)

  • Genetic disorder caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene (deletion of phenylalanine at position 508, denoted by ΔF508).

  • Symptoms: Thick mucus build-up in lungs leading to chronic infections and respiratory issues, digestive problems due to mucus obstruction.

  • Treatment focuses on managing symptoms; includes CFTR modulators to improve protein function.

Protein Import Mechanisms

Co-translational Import

  • Involves signal recognition particles (SRP) that halt translation to target the protein to the ER. This pathway is vital for ensuring proteins folded properly and reach their designated compartments within the cell.

Mitochondrial and Chloroplast Targeting

  • Proteins synthesized in the cytoplasm that contain signal sequences (at their N-terminus) target them to mitochondria or chloroplasts.

  • The Tom complex facilitates their translocation into mitochondria.