10) Cell Trafficking

• Translation takes place in ribosomes at the cytosome of the cell

  Proteins are then sent to organelles using:

  • Signalling Sequences (barcodes)
  • post-translational modifications

  
• Post-Translational Modifications in cell trafficking:

  • Occur after protein synthesis
  • ⤷ Include processes like phosphorylation, acetylation which change the amino acid side chain so it can arrive to its destination

  
• Signal Sequences in cellular trafficking:

  • Short Amino acid sequences at the N/C-terminus of proteins
  • N-Terminal: Positively charged + Hydrophilic
  • C-Terminal: Marks cleavage site of sequence
  • Signal sequences are recognised by soluble or membrane bound receptors, guiding them to their destination
  • In the mitochondria/ER, receptors for the signal sequences are found within the membranes of the organelles

  
• Types of Signal Sequences:

  • Cytoplasmic Proteins - default
  • Secreted Proteins - ER import signal sequence then secreted out of cell
  • Nuclear Proteins - ER import signal sequence and transported to nucleus
  • Cell surface MB proteins - ER signal sequence and Transmembrane domain
• Nuclear Pore Complex (NPC)

  • Nuclear envelop has a double membrane with nuclear pores that allow selective diffusion
  • Import of protein requires nuclear localisation sequences (NLS), Exporting requires NES
  • These attach to ”Importins“ which import cargo to nucleus

  
• Membrane-Bound Proteins:

  Transmembrane proteins are incorporated into the ER membrane when synthesised

  • The protein has a hydrophobic sequence (called the transmembrane domain), which anchors the protein, fixing it into the membrane.

  
• Non-membrane bound proteins:

  • Contain no hydrophobic transmembrane sequence, making it soluble and able to cross the ER membrane
  • The signal peptide sequence is cleaved by signal peptidase and degrade in the membrane
  • A protein plug then closes the inactive channel so the protein remains inside the lumen of the ER!

  
• Movement of Intracellular materials and proteins in organelles

  • Vesicles from one organelle (containing materials/proteins) bud from one membrane and fuses with another with the help of SNAREs
• Exocytosis:

  1. constitutive: Vesicles are continuously released from the cell regardless of external signals - Often used to get rid of substances in the cell
  2. Regulated: Vesicles are released in response to specific signals or stimuli - Often used in secretory cells such as hormones or neurotransmitters

  
• Endocytosis:

  • Requires ATP
  • Uses vesicles to move material into the cell by fusing with lysosomes
  • ⤷ Shapes the membrane into a bud and captures molecules for transport into other organelles
  • Lysosomes (containing lysosymes) hydrolyse material at an acidic pH (using H+ ATPase in the lumen)