When a protein is synthesized on fixed ribosomes, it is threaded into the lumen of endoplasmic reticulum (ER).
Every eukaryotic cell has an endoplasmic reticulum (ER). The ER is arranged systematically into a web-like maze of radiating tubules and flattened sacs stretching all over the cytosol.
The tubules and sacs are all believed to interlink, so that the ER membrane forms an extended sheet bordering a single internal space. This highly complicated space is known as the ER lumen.
The ER traps selected proteins from the cytosol during their synthesis.
The two kinds of proteins include:-
- transmembrane proteins, which are only partially moved across the ER membrane and are submerged in it, and
- water-soluble proteins, which are completely transported across the ER membrane and are discharged into the ER lumen.
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