flügeltürer

Rough Endoplasmic Reticulum
  • Rough ER (RER) is involved in some protein production, protein folding, quality control and despatch
  • Called ‘rough’ because studded with ribosomes
  • Smooth ER (SER) is associated with the production and metabolism of fats and steroid hormones
  • Smooth because no ribosomes and is associated with smooth, slippery fats.

STRUCTURE

  • Continuous membrane of flattened sacs (cisternae) and network tubules, touching nuclear membrane. 
  • Membrane bound ribosomes firmly attached to the outer cytosolic side of the RER
  • However these are constantly being bound and released - will only bind when specific protein-nucleic acid complex forms in cytosol 

FUNCTION

  • Proteins are made by the ribosomes on the surface of the RER - translation
  • Then (some) are threaded inside RER to be modified and transported
  • RER working with membrane bound ribosomes takes polypeptides and amino acids from the cytosol and continues protein assembly including, at an early stage, recognising a ‘destination label’ attached to each of them. 
  • Proteins are produced for the plasma membrane, Golgi apparatus, secretory vesicles, lysosomes, endosomes and the ER. 
  • Some  proteins into the lumen (inside) of the RER; others are processed in RER membrane itself
  • Lumen: some proteins have sugar groups added to form glycoproteins; some have metal groups added
  • EG: in RER four polypeptide chains are brought together to form haemoglobin.

Protein folding unit
lumen of the rough ER: proteins folded to produce biochemical architecture which will provide ‘lock and key’ and other recognition and linking sites.

Protein quality control section

  • Lumen: incorrectly formed or incorrectly folded proteins rejected
  • Rejects stored in the lumen or sent for recycling for eventual breakdown to amino acids. 
  • A form of cystic fibrosis = missing single amino acid, phenylanaline, in a particular position in the protein construction. Quality control section spots the error and rejects, however individual would have been better off with poor product than none at all

From Rough ER to Golgi
In most cases proteins are transferred to the Golgi apparatus for ‘finishing’. They are conveyed in vesicles or possibly directly between the ER and Golgi surfaces. After ‘finishing’ they are delivered to specific locations.