How do the endoplasmic reticulum and Golgi apparatus work together to sort proteins?

The endoplasmic reticulum (ER) and the Golgi apparatus function as a coordinated system for the synthesis, modification, and distribution of proteins within the cell.12

Protein Synthesis and Transport

The process begins in the rough endoplasmic reticulum (RER), where ribosomes synthesize proteins destined for secretion, the cell membrane, or other organelles. As these proteins are synthesized, they enter the ER lumen, where they undergo initial folding and glycosylation—the addition of sugar chains. Once processed, these proteins are packaged into transport vesicles that bud off from the transitional region of the ER and move through the cytoplasm to the Golgi apparatus.3452

Modification and Sorting

Upon reaching the Golgi apparatus, the vesicles fuse with the "cis" face, delivering their contents into the organelle's lumen. As proteins progress through the Golgi’s distinct compartments—from the cis to the medial and finally the "trans" face—they undergo further chemical modifications, such as the trimming or addition of sugar groups and the cleavage of amino acids by enzymes.25

Finally, the trans Golgi network sorts these mature proteins and packages them into vesicles for delivery to their specific destinations, including lysosomes, the cell membrane, or the extracellular environment via exocytosis.12